The National Hydrography Dataset National Hydrography Dataset ... Reach code ... Encoding flow relations among transport and coastline reaches

February 2000

The National Hydrography Dataset Concepts and Contents

Contents

Overview ....................................................................................................................................................... 4

Features ......................................................................................................................................................... 5

Feature types and characteristics............................................................................................................... 5 Encoding feature types and characteristics............................................................................................. 6

Delineation rules ........................................................................................................................................ 7

Common identifier ...................................................................................................................................... 8

Special feature types: Artificial Path, Connector, and Underpass............................................................. 8

Reaches.......................................................................................................................................................... 9

Reach types and delineation....................................................................................................................... 9 Transport reach....................................................................................................................................... 9

Delineation ....................................................................................................................................... 10 Underlying feature rule................................................................................................................. 10 Confluence-to-confluence rule ..................................................................................................... 10 Branched path rule........................................................................................................................ 11 Application of, and deviation from, the rules ............................................................................... 11

Coastline Reach.................................................................................................................................... 11 Waterbody reach................................................................................................................................... 12 Shoreline reach ..................................................................................................................................... 12

Reach code ............................................................................................................................................... 12

Common identifier .................................................................................................................................... 13

Reach summary ........................................................................................................................................ 13

Encoding flow relations among transport and coastline reaches ........................................................... 13

Direction of flow using flow relations ...................................................................................................... 14

Sequencing flow relations along a reach.................................................................................................. 17

Identifying level paths through the drainage network ............................................................................ 17

Stream level .............................................................................................................................................. 18

Tracing stream levels among reach flow relations................................................................................... 19

Metadata and digital update units ............................................................................................................ 21

Digital update units .................................................................................................................................. 21

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Geographic names ...................................................................................................................................... 23

Characteristics of domestic geographic names ........................................................................................ 25

Entry conventions for geographic names ................................................................................................. 25

Common identifier ..................................................................................................................................... 26

Coordinates and related measures ............................................................................................................ 27

Horizontal coordinate referencing system................................................................................................ 27

Lengths and areas .................................................................................................................................... 27

Elevations of water surfaces..................................................................................................................... 27

Data Quality................................................................................................................................................ 29

Conterminous United States (excluding the Pacific Northwest), Hawaii, Puerto Rico, and the Virgin Islands ...................................................................................................................................................... 29

Lineage ................................................................................................................................................. 30 Information Sources ......................................................................................................................... 30 Processes .......................................................................................................................................... 31

Convert DLG data to features....................................................................................................... 31 Build reaches ................................................................................................................................ 31

Attribute accuracy ................................................................................................................................ 32 Logical consistency .............................................................................................................................. 33 Completeness........................................................................................................................................ 33 Horizontal positional accuracy ............................................................................................................. 34 Vertical positional accuracy ................................................................................................................. 34

Pacific Northwest and Alaska................................................................................................................... 34

Glossary....................................................................................................................................................... 35

References ................................................................................................................................................... 41

Appendix A. Overview of features, reaches, and related items. ............................................................ 42

Appendix B. Feature code and description field structures and definitions ........................................ 45

Feature code structure ............................................................................................................................. 45

Description field structure........................................................................................................................ 45

Appendix C. Encoding characteristics using field names ...................................................................... 55

Appendix D. Development of Reach Files and related concepts............................................................ 59

Purpose and approach ............................................................................................................................. 59

Reach file development............................................................................................................................. 59

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Appendix E. Transport reach delineation rules and examples.............................................................. 61

Underlying feature rule ............................................................................................................................ 61 Exception: Insignificant lake/pond features (10-acre rule)................................................................... 62

Confluence-to-confluence rule ................................................................................................................. 63 Like feature types (and their surrogates) .............................................................................................. 63

Exception: Insignificant confluences (5-mile rule)........................................................................... 65 Unlike feature types: stream/river and canal/ditch ............................................................................... 66 No confluence: underpasses and pipelines ........................................................................................... 66

Branched path rule ................................................................................................................................... 67 Exception: Insignificant lake/pond features (10-acre rule)................................................................... 68

Appendix F. Organization and examples of hydrologic units................................................................ 69

Appendix G. Peculiarities ......................................................................................................................... 72

Quad Edge Effects .................................................................................................................................... 72

DLG-3 coding inconsistencies.................................................................................................................. 72 Inland oceans........................................................................................................................................ 72 Disguised aqueducts ............................................................................................................................. 72 Coastal features - foreshore versus sand.............................................................................................. 73 Stream/rivers controlled by dams that become a series of slackwater pools ........................................ 73 Reservoir versus lake/pond................................................................................................................... 73

NHD linework doesn’t match the published USGS 1:100,000-scale map................................................ 73

Names ....................................................................................................................................................... 73

Subbasin (formery known as cataloging unit) boundaries and the features that touch and cross them... 73

Squared-off coastal subbasin boundaries................................................................................................. 74

Empty subbasins ....................................................................................................................................... 74

Coastline reaches that bound stream/rivers ............................................................................................. 74

Waterbody reaches ................................................................................................................................... 75

Flow/coordinate direction/measure direction .......................................................................................... 75

Artificial Paths that fall outside of the 2-D features they represent ......................................................... 75

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Overview The National Hydrography Dataset (NHD) is a comprehensive set of digital spatial data that encodes information about naturally occurring and constructed bodies of water, paths through which water flows, and related entities. The information encoded about these features includes classification and other characteristics, delineation, geographic name, position and related measures, a "reach code" through which other information can be related to the NHD, and the direction of water flow. In addition to this geographic information, the dataset contains metadata and information that supports the exchange of future updates and improvements to the data.

The data support many applications, such as: • Making maps. Positional and descriptive data in the NHD provide the starting point for making

many different kinds of maps. • Geocoding observations. Much like street addresses provide a way to link data to a road network,

the NHD's "reach code" provides the means to link data to water features. • Modeling the flow of water along the Nation's waterways. Information about the direction of

flow, when combined with other data, can help users model the transport of materials in hydrographic networks, and other applications.

• Maintaining data. Many organizations would like to share the costs of improving and updating their collections of geographic data. Unique identifiers and other methods encoded in the NHD help to solve technical problems of cooperative data maintenance.

In 1999, coverage was made available for the conterminous United States and Hawaii. Data for Puerto Rico, the Virgin Islands, and parts of Alaska will follow. The production of data for the remainder of Alaska will be ongoing for several years. Efforts to maintain and improve the data will occur continually.

The NHD is the culmination of cooperative efforts of the U.S. Environmental Protection Agency (USEPA) and the U.S. Geological Survey (USGS). Other organizations also contributed to the effort.

This volume describes the concepts and information content of the NHD, including features, reaches, metadata, geographic names, coordinate systems and related measures, and data quality.

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Features

A feature is a defined entity and its representation. In the NHD, features include naturally occurring and constructed bodies of water, paths through which water flows, and related entities. Features are classified by type, may be described by additional characteristics, and are delineated using standard methods.

Feature types and characteristics Features are classified by type. These feature types, such as "stream/river", "canal/ditch", and "lake/pond", provide the basic description of the features. Each type has a name and a definition. For example, the three most frequently encountered feature types and corresponding definitions are as follows:

Feature Type Definition STREAM/RIVER A body of flowing water. LAKE/POND A standing body of water with a predominantly natural shoreline surrounded by land. CANAL/DITCH An artificial open waterway constructed to transport water, to irrigate or drain land, to connect

two or more bodies of water, or to serve as a waterway for watercraft.

Characteristics, which are traits, qualities, or properties of features, are provided for many feature types. Each characteristic has a name, a definition, and a list of values and corresponding definitions. For example, the features lake/pond and stream/river have the characteristic Hydrographic Category:

Name Definition Characteristic Hydrographic Category Portion of the year the feature contains water.

Values Intermittent Contains water for only part of the year, but more than just after rainstorms and at snowmelt.

Perennial Contains water throughout the year, except for infrequent periods of severe drought.

Appendix A lists the names and characteristics associated with each feature type. The "Standards for National Hydrography Dataset" (USGS, 1999) contains the names and definitions of all feature types, characteristics, and values. The document is available online through http://mapping.usgs.gov/standards/.

Main points: • Features in the NHD include naturally occurring and constructed bodies of water, paths

through which water flows, and related entities. • Features are classified by feature type. • Features may also be described by characteristics. These characteristics are encoded

using three methods: a feature code, a description field, and a series of fields. • Features are delineated using points, lines, or areas. For linear features for which the

direction of water flow is known, the lines are oriented in the direction of flow. Along the coastline, the lines usually are oriented so that the sea or ocean is to the right of the direction of the line.

• The common identifier uniquely identifies the occurrence of each feature. • Features of the types known as artificial path, connector, and underpass serve

special functions.

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Encoding feature types and characteristics A five-digit feature code encodes the feature type and combinations of characteristics and values that can be assigned to a type. The first three digits encode the feature type, and the last two digits encode a set of characteristics and values. For example, the feature type "dam/weir" has the code "343". There are five combinations of characteristics and values that can be assigned to features of this type. These combinations are assigned the values of "00" through "04". The resulting possible feature codes are listed below:

Feature Code Feature Type Characteristic | Value

34300 34301 34302 34303 34304

DAM/WEIR DAM/WEIR DAM/WEIR DAM/WEIR DAM/WEIR

Feature type only: no attributes Construction Material|earthen; Operational Status|operational Construction Material|earthen; Operational Status|under construction Construction Material|nonearthen; Operational Status|operational Construction Material|nonearthen; Operational Status|under construction

Feature codes are stored in a data element named "FCODE".

For those who prefer to use text instead of the numeric code, words also are used to encode the feature type, characteristic, and value information in the feature code:

• Feature types are encoded with a character string (for example, "Stream/River") in a data element named "FTYPE".

• Two approaches, which format the same information in different ways, encode characteristics and values using words:

1. A description field, which holds a character string that contains all characteristics and values associated with a feature code. The description field is encoded in a data element named "DESCRIPT".

2. A text field for each characteristic associated with a feature code. The name of each field is an abbreviation of the name of the characteristic.

For example, a feature classified as a "Dam/Weir" may have the characteristic of "Construction Material" with a value of "earthen", and the characteristic of "Operational Status" with a value of "operational". This information is encoded as follows:

Information Encoded

Feature Type, Characteristics,

and Values Feature

Type Characteristics and Values

Method Feature Code field

Feature Type field Description field Field for each Characteristic

Field Name FCODE FTYPE DESCRIPT COM OPS

34300 DAM/WEIR feature type only: no attributes

34301 DAM/WEIR Construction Material|earthen; Operational Status|operational Earthen Operational Examples

of Values

34302 DAM/WEIR Construction Material|earthen;

Operational Status|under construction

Earthen Under construction

Appendix B lists each feature code and its corresponding description. Appendix C lists the name of the field for each characteristic and the list of values for each characteristic.

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A feature type and feature code are assigned to each feature. Description fields, fields for each characteristic, and feature codes are encoded in a lookup table. Associating the lookup table with features by matching the feature codes allows words denoting characteristics and values to be substituted for the numeric feature code.

Delineation rules The shape and extent of features are delineated using points (including nodes), lines, or areas (see figure 1).

Figure 1. Features are delineated using points, lines, or areas.

The delineation of each feature follows three rules:

1. The delineated feature must be contiguous. 2. The delineated feature must have the same dimensionality; that is, it must be one point, one or

more lines, or one or more areas. 3. The delineated feature can have only one feature type and must have the same set of

characteristics and choices of values throughout its extent (including values for geographic name, surface elevation, and metadata, which are discussed in later sections).

Delineations of linear and areal features of different types may overlap. Where they overlap, they use the same lines or areas for their delineations. For example (see figure 2) the delineation of linear features of the types canal/ditch and bridge use the same lines where they overlap. Similarly, the delineations of areal features of the types lake/pond and swamp/marsh share the same areas where they overlap.

Figure 2. Examples of overlapping delineations of features.

SPRING/SEEP (Point)

STREAM/RIVER (Line)

INUNDATION AREA(Area)

LAKE/POND(Area)

SUBMERGED STREAM(Area)

ARTIFICIAL PATH(Line)

STREAM/RIVER(Area)

Lake/pond

Swamp/marsh

Overlap (area shared for delineation)

Canal/ditch Bridge

Overlap (line shared for delineation)

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Features delineated with lines have two additional rules: they may not have branches, and they must start and stop at decision or merge points along a network. These points exist where a path represented by a network can branch among two or more choices. For example, a decision point exists at the confluence of two stream/rivers; at the confluence, one can choose among two or more paths. Conversely, a decision point does not exist where features at different elevations cross; travel along the path of each feature is independent from that of the other. For example, a decision point does not exist where a canal/ditch passes over a stream/river.

Lines always have a direction; that is, lines trace a path between places where they start and stop. This characteristic provides a means to encode the direction of the flow of water. For features delineated with lines, for which the direction of flow is a prominent characteristic (the feature types artificial path, canal/ditch, connector, pipeline, and stream/river), and for which the direction of flow is known, the lines are oriented in the direction of the flow of water. Note that the direction of flow is not always known (for example, where source materials are ambiguous) or uniform (for example, in tidal areas), and so the lines are not always oriented in the direction of flow. In addition, along the coastline of the United States, the lines are oriented so that the water is to the right of the direction of the line.

The delineation of features stops at the borders of the United States.

Common identifier The common identifier is a 10-digit integer value that uniquely identifies the occurrence of each feature. Each value occurs only once throughout the Nation. Once assigned, the value is associated permanently with its feature. When a feature is deleted, the value for its identifier is retired. The common identifier is stored in a data element named "COM_ID".

Special feature types: Artificial Path, Connector, and Underpass The feature types artificial path, connector, and underpass serve special functions. The artificial path and connector ensure that the hydrographic network is complete. The artificial path represents the flow of water into, through, and out of features1 delineated using areas (that is, it serves as a centerline) and also delineates the coastline. The connector fills gaps in the delineation of other features.

An underpass and two relations ("above" and "below"), represent places where features cross at different elevations. An example is where a feature of the type canal/ditch passes over a feature of the type stream/river (see figure 3.) The canal/ditch is encoded as being above an underpass, and the stream/river is encoded as being below the (same) underpass.

Common Identifier Feature Type Is Above Identifier Is Below Identifier<Common Identifier forthe Underpass> UNDERPASS <Common Identifier for the

Canal/Ditch><Common Identifier for theStream/River>

1 These feature types are area of complex channels, estuary, ice mass, lake/pond, playa, reservoir, swamp/marsh, wash, and wide canal/ditch and stream/river.

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Figure 3. Encoding an underpass.

Appendix A identifies the feature types that may be related to an underpass. Note that underpasses are encoded only where they can be observed from source materials.

Reaches

A reach is a continuous, unbroken stretch or expanse of surface water. In the NHD, this idea has been expanded to define a reach as a significant segment of surface water that has similar hydrologic characteristics, such as a stretch of stream/river between two confluences, or a lake/pond. Reaches also are defined for unconnected (isolated) features, such as an isolated lake/pond.

Once a reach is defined for a segment of water and assigned a reach code, the reach will rarely be changed, if at all. Many activities for improving and updating the data, such as the integration of more accurate coordinate data, the replacement of linear feature delineations with areas, or the addition of smaller features, change only the alignment of existing reaches and do not require that they be redefined.

The stability of reach definition and reach code assignment makes reaches a useful foundation for geocoding observations and statistics. Changes to the surface waters (for example, the creation of a new reservoir) and corrections to erroneous delineations of reaches, of course, would change reaches and reach codes.

The NHD is the latest refinement of reaches and reach codes. Information about earlier implementations is in Appendix D

Reach types and delineation Three types of reaches are in use: transport, coastline, and waterbody reaches. A fourth type, shoreline reach, has not been implemented.

Transport reach A transport reach represents the pathway for the movement of water through a drainage network. These reaches also are used to encode the direction in which water flows along the reach when the direction is known. They provide a basis on which locations of observations can be geocoded and linked to the drainage network.

Main points: • Reaches are segments of surface water with similar hydrologic characteristics. • Three types of reaches are in use: transport, coastline, and waterbody reaches. A

fourth type, the shoreline reach, has not been implemented. • Transport and coastline reaches are based on linear features. For transport reaches

for which direction of water flow is known, the lines are oriented in the direction of flow. For coastline reaches, the lines usually are oriented so that the water is to the right of the reach.

• Waterbody reaches are based on areal features, most often of the type lake/pond. • A reach code uniquely labels each reach. • The common identifier uniquely identifies the occurrence of each reach.

Underpass Canal/Ditch Stream/River

Canal/Ditch

Underpass

Stream/River

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Delineation Lines delineate transport reaches. Only lines that delineate features of the types canal/ditch, pipeline, stream/river, artificial path, and connector delineate transport reaches. For transport reaches for which the direction of flow is known, the lines are oriented in the direction of the flow of water. Note that the direction of flow is not always known (for example, where source materials are ambiguous) or uniform (for example, in tidal areas), and so some lines are not oriented in the direction of flow.

Three general rules determine the location of the ends of transport reaches: the underlying feature rule, the confluence-to-confluence rule, and the branched path rule.

Underlying feature rule The delineation of a transport reach follows that of one or more features. Where two or more features are followed, a transport reach follows delineations of :

• features of the same type (for example, exclusively stream/river) or • features of the same type, in combination with those of types artificial path and/or connector. For

an artificial path to be included, the areal feature through which the artificial path is delineated must be of the same type as the linear feature with which the artificial path makes up the reach. For example, the delineation of a reach may follow an artificial path through an areal canal/ditch and a contiguous linear canal/ditch.

A transport reach always follows the entire delineation of the underlying feature or features; the delineation of a feature is not split among reaches. Transport reaches abut and do not overlap.

Confluence-to-confluence rule In the confluence-to-confluence rule, a transport reach is a stretch of water between:

• confluences, • a head and a confluence, • a confluence and a mouth or other terminus, or • a head and a mouth or other terminus.

In the application of this rule, divergences serve the same role as confluences. Reaches defined by this rule must be contiguous and may not branch (see figure 4).

Figure 4. Confluence-to-confluence reach delineation.

Note that some confluences are not considered to be significant enough to break the delineation of a reach. Thus, although transport reaches start and end at confluences, not every confluence causes a transport reach to start or end (see Appendix E for more information).

1A

3C

2B

5E

4D

Confluence

Reach

Confluence-to-confluence rule REACH 1 follows STREAM/RIVER A. REACH 2 follows STREAM/RIVER B. REACH 3 follows STREAM/RIVER C. REACH 4 follows STREAM/RIVER D. REACH 5 follows STREAM/RIVER E.

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Branched path rule A branched path transport reach connects reaches that enter and exit an areal feature (see figure 5). Reaches that follow this rule occur most often in large2 features of type lake/pond and swamp/marsh, and they also may occur in other areal features. Artificial paths delineated within the areal feature provide the lines needed to delineate this special transport reach. The reach may branch and at times be discontiguous.

Figure 5. Branched path reach delineation.

The branched path transport reach avoids the need to define flow channels, confluences and divergences, and confluence-based transport reaches in areal features. It is used where the information needed to delineate these items reliably is not available, and at other places.

Application of, and deviation from, the rules The underlying feature, confluence-to-confluence, and branched path rules govern the delineation of most transport reaches. These rules, however, have exceptions. Unusual configurations of features require modification of the rules, as do the variable condition and ambiguities of information sources. In places where unusual configurations of features or ambiguities in sources occur more often, a larger percentage of reaches delineated using modified rules will be found. Appendix E lists more examples of reach delineation rules and exceptions to the rules.

Coastline Reach A coastline reach represents a section of coastline along the Atlantic, Pacific, or Arctic Oceans, the Great Lakes, the Gulf of Mexico, or the Caribbean Sea. These reaches provide a basis for geocoding locations of observations along the coastline.

Artificial paths that follow the coastline provide the lines used for coastline reach delineation. The delineation of a coastline reach may follow one or more artificial paths. A coastline reach always follows the entire delineation of the underlying artificial path or paths; the delineation of an artificial path is not split among reaches. Coastline reaches abut and do not overlap.

The lines are usually oriented so that the water is to the right of the line. This results in a general orientation of coastline reaches northward along the Atlantic Ocean, southward along the Pacific Ocean, eastward along the Gulf of Mexico, westward along the Arctic Ocean and the U.S. side of the Great Lakes, and counterclockwise around islands. 2 Large areal features cover 10 or more acres (approximately 4.05 or more hectares).

2F

3G 4H

1A

1D

1B

5I

1C

1E

Feature with an area of 10 acres or larger.

Branched path reach

Other reaches

Branched path rule REACH 1 follows ARTIFICIAL PATHS

A, B, C, D, and E. Confluence-to-confluence rule REACH 2 follows STREAM/RIVER F. REACH 3 follows STREAM/RIVER G. REACH 4 follows STREAM/RIVER H. REACH 5 follows STREAM/RIVER I.

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A coastline reach is delineated for coastal islands that have a drainage network or more than 5 miles (approximately 8.06 kilometers) of isolated drainage. Other coastal islands may or may not be delineated with a coastline reach.

The ends of coastline reaches occur where transport reaches discharge into the oceans, Gulf of Mexico, Caribbean Sea, or Great Lakes (although not every point of discharge is the end of a coastline reach). At the mouth of an areal stream/river, coastline reaches end where the artificial path used to delineate the transport reach intersects the coastline.

Waterbody reach A waterbody is a hydrographic feature delineated using areas. Reaches assigned to waterbodies are termed waterbody reaches. These reaches provide a means to geocode observations for areas of water. (In contrast, transport reaches represent the path of a flow of water and provide a means of geocoding observations along the path.) Areal delineations of features provide the areas used to delineate waterbody reaches3.

Shoreline reach The shoreline reach has not been implemented and is discussed for information purposes only. A shoreline reach would represent all or part of the shoreline of an inland waterbody. Analogous to coastline reaches, they would provide a basis for geocoding locations of observations along the shoreline. Lines would delineate shoreline reaches. These reaches would abut and not overlap. The types of features that would provide these lines are being investigated, as are questions of the direction, relative to the water, in which these reaches should be ordered, means of handling islands, and rules for deciding where these reaches would start and stop.

Reach code A reach code is a numeric code that uniquely labels each reach. This 14-digit code has 2 parts: the first 8 digits are the hydrologic unit4 code for the subbasin in which the reach exists; the last 6 digits are assigned in sequential order, and arbitrarily among the reaches.

Each reach code occurs only once throughout the Nation. Once assigned, a reach code is associated with its reach permanently. If a reach is deleted, its reach code is retired. A reach code should not be altered.

Reach codes can serve to geocode an observation to a reach or a position along a reach. Observations can be geocoded to an entire reach by associating the reach code with the observation data, or to sections of a transport, coastline, or (the planned) shoreline reach by using the reach and reach code as the basis of a linear referencing system.

Reach codes are stored in data elements named "RCH_CODE". In addition to the reach code, the date on which the code was assigned in the NHD is encoded. The date is stored in data elements named "RCH_DATE".

3 Features of the type lake/pond most often are the basis for waterbody reach delineation in the initial release. An exception is the State of Washington where, on the basis of work done by a State agency, waterbody reaches are based on areal delineations of the features types ice mass, lake/pond, reservoir, and swamp/marsh. In the future, feature types that could provide the basis for waterbody reach delineation are area of complex channels, estuary, lake/pond, playa, sea/ocean, wash, and areal delineations of canal/ditch and stream/river.

4 Hydrologic units (Seaber, Kapinos, and Knapp, 1987) provide a systematic means of identifying and subdividing river-basin units. The subbasin is the fourth-level subdivision of hydrologic units. Additional information about hydrologic units is in appendix F.

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Common identifier In addition to identifying each feature, the common identifier uniquely identifies each reach. Each 10-digit integer value occurs only once throughout the Nation. Once assigned, the value is associated permanently with its reach. When a reach is deleted, the value for its identifier is retired. The common identifier is stored in a data element named "COM_ID".

Reach summary Table 1 summarizes the types, related items, delineation, and underlying features of reaches.

Table 1. Summary of reach organization by type.

Reach Type Related Information Delineation Features Types That Provide Delineation

Coastline Reach Code Reach Code Assignment

Date

Line Artificial Path

Shoreline This reach type has not been implemented. Transport Reach Code

Reach Code Assignment Date

Line Artificial Path, Canal/Ditch, Connector, Pipeline, Stream/River

Waterbody Reach Code Reach Code Assignment

Date

Area Only Lake/Pond for the initial release in most places; Ice Mass, Lake/Pond, Reservoir, and Swamp/Marsh in the initial release for Washington State.

Encoding flow relations among transport and coastline reaches

Flow relations among transport and coastline reaches encode drainage network connectivity among reaches independently from their delineations. Relations among transport reaches define a connected hydrographic network and encode the direction of water flow among reaches. This connectivity enables hydrologic sequencing of reaches (what is upstream and downstream of a given point in the hydrographic network) and navigating the network in an upstream or downstream direction. Similarly, relations among coastline reaches allow the traversal of the coastline using reaches. Relations among transport and coastline reaches connect hydrographic networks to the coastline and enable the ordering of the mouths of these networks along the coastline. Flow relations are especially useful in applications that require information about connectivity among, but not about, the underlying delineations and coordinate positions of reaches.

Flow relations are encoded by identifying a pair of reaches that touch and describing the flow of water between the reaches. Note that the direction of flow is not always known (for example, where source materials are ambiguous); in these cases, flow relations are not created.

Main points: • By using flow relations to relate transport and coastline reaches, surface waters are

modeled as a connected network. • Flow relations encode:

• network connectivity among reaches. • the direction of water flow between transport reaches. • the traversal of the coastline along coastline reaches. • the order of termini of drainage networks along the coastline.

• Sequence numbers order flow relations of transport reaches along the interior of a transport or coastline reach.

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Direction of flow using flow relations The common identifiers for a pair of reaches and a description of the direction of water flow between the reaches are used to encode flow relations. Five values describe the direction of water flow between transport reaches; a sixth value describes connections between pairs of touching coastline reaches or between touching transport and coastline reaches:

1. in — the first reach flows into the second reach at the top of, or at an interior point along, the second reach. (See Figure 6, Figure 7, Figure 8, and Figure 9.) Most flow relations between transport reaches are of this type.

2. out — the first reach flows from the second reach from an interior point somewhere along the second reach. (See Figure 7.)

3. bidirectional — the first and second reaches touch end-to-end, or the first reach touches an interior point along the second reach, and flow can occur in either direction. This value has not been implemented.

4. network start — the second reach starts at a head of the drainage network. (The field for the first reach is null.) (See Figure 6.)

5. network end — the first reach ends at a terminus, other than at the coastline, of a drainage network. (The field for the second reach is null.) (See Figure 6.)

6. non-flowing connection —

a) between coastline reaches. Coastline reaches are directed so that water is to their right. The flow relation between coastline reaches is ordered so that the end of a coastline reach (the first reach in the relation) connects to the beginning of, or to some point along, the next coastline reach (the second reach in the relation) that continues the traversal of the coastline, with water to the right of the traversal. Flow relations between coastline reaches allow the coastline to be traversed. (See the flow relation between coastline reaches 1 and 4 in Figure 9.)

b) between a transport and coastline reach. The transport reach connects to the coastline reach that continues the traversal of the coastline from the place where they touch. The flow relation is ordered so that the end of the transport reach (the first reach in the relation) connects to the beginning of, or at some point along, the coastline reach (the second reach in the relation) that continues the traversal of the coastline. Relations between coastline and transport reaches allow the order in which transport reaches intersect the coastline to be computed. (See the flow relation between transport reach 3 and coastline reach 4, and transport reach 6 and coastline reach 1, in Figure 9.)

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Figure 6. Flow relations illustrating in, out, network start, and network end directions. (A common identifier value of "0" represents a null entry.)

Figure 7. Flow relations illustrating out and in directions.

Reach Direction of flow

1

3

4

2

5

Flow Relations

Common Identifier for First Reach

Common Identifier for

Second Reach Direction

Description

1 3 In 1 4 In 2 1 Out 5 3 Out 5 3 In

Flow Relations Common

Identifier for First Reach



Description

1 2 In 4 2 In 6 5 In 5 3 In 2 3 In 0 1 Network Start 0 4 Network Start 0 6 Network Start 0 5 Network Start 3 0 Network End

1

3 2

4

6

5

Reach

Head Terminus

Direction of flow

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Figure 8. Flow relations for a branched path reach.

Figure 9. Flow relations illustrating non-flowing connections. (A common identifier value of "0" represents a null entry.)

Flow relations are stored as four data elements. The common identifier for the first reach is stored in a data element named "COM_ID_1". The common identifier for the second reach is stored in a data element named "COM_ID_2". The direction description is stored in two data elements: as a character string in a data element named "DIR_TEXT" and as a numeric alias5 in a data element named "DIRECTION".

5 These are the numeric codes for the direction descriptions:

"709" for In. "710" for Out. "711" for Bidirectional. "712" for Network Start. "713" for Network End. "714" for Non-flowing Connection.





Description

2 1 In 3 1 In 4 1 In 1 5 In

2

3 4

5

1

Waterbody

Branched reach

Other reaches Direction of flow





Description

1 4 Non-flowing Connection


2 3 In 5 3 In 0 6 Network Start 6 1 Non-flowing

Connection

Transport reach Coastline reach Direction of flow

Ocean, Great Lake

3 2

5

4

6

1 1 (cont’d)

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17

Sequencing flow relations along a reach Sequence numbers order flow relations of transport reaches that flow into or out of the interior of another transport reach. The reach that flows in or out is always the first reach in the flow relation; the reach along whose interior the inflowing and outflowing reach is being sequenced is the second reach in the flow relation. The flow relations are numbered sequentially, starting with 1, from upstream to downstream along the interior of the second reach.

Sequence numbers also order flow relations of transport reaches that have a non-flowing connection to the interior of a coastline reach. The non-flowing connection flow relations of intersecting transport reaches are numbered sequentially, starting with 1, from start to end along the interior of the coastline reach.

A sequence number of 0 means that the first reach touches the second reach at its end and not along its interior. The sequence number is stored with the flow relations in a data element named "SEQUENCE".

Figure 10 illustrates the sequencing of flow relations.

Figure 10. Sequencing flow relations along transport and coastline reaches. (A common identifier value of "0" represents a null entry.)

Identifying level paths through the drainage network

Transport reaches and flow relations provide the components of the drainage network. The level path builds on these elements to create a sequence of transport reaches that trace the main stem for a given flow of water. The level path traces the flow between a head and the next largest flow of water or between a head and the ocean. Geographic names often follow these paths.





Description Sequence

1 3 In 0 2 3 In 0 5 3 Out 1 5 3 In 3 4 3 In 2 3 8 Non-flowing

Connection 0

0 6 Network Start

0


1


0

Coastline reach Transport reach

Direction of flow

nFlow relation sequence number

Ocean, Great Lake

2

3

5 4

1

0

1

2

3

3 (cont’d)

08 (cont’d) 8

6

17

Main points: • A level path is a sequence of transport reaches that traces the main stem for a

given flow of water. • Stream level identifies the main path to which a transport reach belongs. • The delta level identifies main paths of water flow among flow relations.

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Level paths are encoded by associating stream levels to transport reaches and by associating a delta level with flow relations.

Stream level The stream level is a numeric code that identifies each main path of water flow through a drainage network. Stream level is assigned by identifying the terminus of a drainage network (see Figure 11). The lowest value6 for stream level is assigned to a transport reach at the end of a flow and to upstream transport reaches that trace the main path of flow back to the head. The stream level value is incremented by one and is assigned to all transport reaches that terminate at this path (that is, all tributaries to the path) and to all transport reaches that trace the main path of the flow along each tributary back to its head. The stream level value is incremented again and is assigned to transport reaches that trace the main path of the tributaries to to their heads. This process is continued until all transport reaches for which flow is encoded are assigned a stream level.

Figure 11. Stream level assignment for a simple drainage network.

For example (see Figure 12), the Mississippi River terminates at the Gulf of Mexico. The transport reaches that trace the main flow of the river, from the head to the mouth on the Gulf, are assigned a stream level of 1. The transport reaches that trace the main flow of each tributary to the Mississippi River (such as the Ohio/Monongahela Rivers) from their heads to their termini on the Mississippi River, are assigned a stream level of 2. The transport reaches that trace the main flow of each tributary to the level 2 tributaries (such as the Tennessee River, which is a tributary to the Ohio/Monongahela Rivers), from each head to each mouth on their level 2 tributary, are assigned a stream level of 3.

6 The lowest value for stream level is:

"1" for transport reaches that terminate at the Atlantic, Pacific, or Arctic Oceans, the Gulf of Mexico, or the Caribbean Sea.

"2" for transport reaches that terminate at the Great Lakes or the Great Salt Lake. "3" for transport reaches that terminate at the boundary of the United States with Canada or Mexico. "4" for transport reaches that terminate at any other place (isolated drainage).

1

2

3

4

Ocean

1

1

2

2

2

3

3

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19

Figure 12. Stream level assignments along the Mississippi River.

Ideally, each main path would trace the flow of the largest volume of water. Stream levels encoded in two predecessors7 to the NHD were based on flow volume data, and these paths were retained wherever possible. Data about flow volumes were not available for other reaches. Instead, the path with the same geographic name was used to determine the level path. Where a geographic name was not available or known, the longest and straightest path was used to determine a main path. Where this rule did not adequately discriminate among the choices, the rightmost (looking upstream) transport reach was assumed to be the continuation of a main path.

Stream level is stored in a data element named "LEVEL". The special value "-9998" means that a value can be applied to the transport reach but has not been specified. This value usually occurs where flow relations cannot be determined or have not been encoded. Without this information, main paths cannot be identified and stream level cannot be assigned. This value is also assigned to coastline reaches.

Tracing stream levels among reach flow relations To help identify main paths in the flow relations table, the difference, or delta level, between the stream levels of inflowing and outflowing reaches is encoded. To calculate this value, the stream level of the second reach in the relation is subtracted from the stream level of the first reach in the relation (see Figure 13).

Delta levels of "0" mean that a relation links two reaches that are on the same main path. In most other cases, the delta level will have a value of "1". Areas of complex divergence and braided stream configurations can yield other values. A special value of "-9999" means that a value for delta level is not applicable to the relation, and this occurs when the direction is bidirectional, network start, network end, or non-flowing connection. A special value of "-9998" means that a value could not be specified because the stream level was not specified for one or both reaches that are associated by the flow relation.

The values are stored with the flow relations in a data element named "DELTA_LVL".

7 Reach File Versions 1 and 2, which are described in Appendix D.

Mississippi(level 1)

Ohio/Monongahela(level 2)

Tennessee (level 3)

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Figure 13. Difference in stream levels encoded with flow relations.





Description Sequence Delta Level

1 3 In 0 1 2 3 In 0 0 5 3 Out 1 1 5 3 In 3 1 4 3 In 2 1 3 8 Non-flowing

Connection 0 -9999

0 6 Network Start

0 -9999


1 -9999


0 -9999

2

3

5 4

1

3 (cont’d)

8 (cont’d) 8

6

7

Transport reach - level 1

Direction of flow Coastline reach - level -9998

Ocean

Transport reach - level 2

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21

Metadata and digital update units

Metadata, or data about data, are data that describe the content, quality, condition, and other characteristics of data. Metadata answer questions such as "How current are these data?"; "How accurate are they?"; "Are there any restrictions on their use?"; "What is their coordinate system?"; and many others. Metadata help organizations manage data, advertise and share data, and make informed use of data.

Metadata for the NHD use data elements from the "Content Standards for Digital Geospatial Metadata" (Federal Geographic Data Committee, 1994). The standard allows the identity, quality, spatial data organization and reference, entity and attribute definitions, distribution sources and forms, and metadata of the data to be documented. The metadata are provided as text files.

For the NHD, a general set of metadata accompanies each set of data. This NHD (for National Hydrography Dataset) metadata provides general information that applies to all data.

Digital update units Within the NHD, values differ for metadata elements such as currentness and quality. For example, features originate from more than 1,800 datasets of different vintages, and reaches from more than 2,100 datasets. In the future, many organizations will collaborate to maintain and improve the data. These new data will differ in currentness, source, accuracy, and other characteristics. A single document that provides metadata for these dynamic and varied data would be either very general or very unwieldy.

Digital update units associate specific metadata information with selected features and reaches. A digital update unit is a collection of one or more features and (or) reaches to which a set of metadata values applies. These values include only those needed to describe unique aspects of the associated features or reaches. A feature or reach may be a member of one or more digital update units.

Metadata associated with digital update units supplement information provided in the NHD metadata. These digital update unit metadata amplify, and in some cases replace, the more general information provided in the NHD metadata.

In the initial release of the NHD, there are two types of digital update units; subbasin and quadrangle digital update units (see Figure 14):

• subbasin digital update units associate reaches with metadata about sources of information and processing steps used to construct reaches and analysts' notes about unique situations encountered in the process.

Main points: • Metadata, or data about data, are data that describe the content, quality, condition, or

other characteristics of data. • A digital update unit is a collection of one or more features and (or) reaches to which

a set of metadata values apply. It provides the means to associate different values for metadata elements with features and reaches.

• In the initial release of the NHD, there are three sets of metadata: 1. a general NHD set of values for metadata elements that apply to all data. 2. cataloging unit digital update units that associate unique values for metadata

elements with sets of reaches. 3. quadrangle digital update units that associate unique values for metadata

elements with sets of features. • Metadata are provided in text files associated with a dataset (for the NHD metadata)

or with a digital update unit (for the cataloging unit and quadrangle metadata).

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• quadrangle digital update units associate features with metadata about the sources of information (including the currentness of the data) and processing steps used to construct features.

Figure 14. Digital update units associate metadata with a set of features and (or) reaches. Metadata associated with each digital update unit amplify or replace information provided as NHD metadata.

As development and improvement of the data continue, additional digital update units will be created. These may have collections of features, reaches, and shapes different from the digital update units in the initial release. These new digital update units also may have metadata elements different from those included in the digital update units in the initial release.

Digital update units are assigned unique identifiers and stored in data elements named "DUU_ID".

3E 2D 2C

1A

1B

4F

5H

5G

5I b

c

a

AFeatures and reaches

Digital update unit for features

Endpoints of features and reaches caused by delineation rules.

Digital update unit for reaches

Additional endpoints of features required to associate features to appropriate metadata.

Digital Update Unit “A”(cataloging unit)

Metadata that apply to reaches 1 through 5.

Digital Update Unit “c”(quadrangle)

Metadata that apply to features A and G.

Digital Update Unit “b”(quadrangle)

Metadata that apply to features B, C, F, and H.

Digital Update Unit “a”(quadrangle)

Metadata that apply to features D, E, and I.

NHD metadata(all features and reaches)

Metadata that apply to all features and reaches.

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Geographic names

A geographic name is "the proper name, specific term, or expression by which a particular geographical entity is, or was, known" (Orth and Payne, 1997, p. 43). Geographic names designated as being official for Federal use are encoded for many reaches and features. These names were taken from the National Geographic Names Database of the Geographic Names Information System8 (USGS, 1995), the Federal Government's primary source for identifying official names.

Reaches and features can carry geographic names (see Figure 15 and Table 2). Reaches carry geographic names most often. If there is no reach to carry a name, then the feature carries the name. Appendix A lists the types of features that may carry names.

Geographic names are stored in data elements named "NAME". In addition, the eight-digit identifier for a name in the Geographic Names Information System is stored in data elements named "GNIS_ID".

Figure 15. Geographic names applied to transport and waterbody reaches.

Note that geographic names have not been applied to all reaches and features. Names are applied to transport and coastline reaches that were named in the USEPA's Reach File version 3 (see Appendix D). Names are also applied to waterbody reaches and features delineated using points or areas in cases where batch processes can reliably associate the name with the reach or feature.

8 Information about the Geographic Names Information System is available online at http://mapping.usgs.gov/www/gnis/.

Main points: • A geographic name is the proper name, specific term, or expression by which a

particular geographic entity is known. • Reaches carry the geographic name most often; features carry names if they do not

underlie a reach.

Bowl of Tears

Tuhare Lakes ConstantineLake

HarveyLake

Whitney Lake

ClevelandLake

Hunky Dory Lake

MulhallLakes

FancyLake

TreasureVault LakeMissouri

Lakes

Sopris Lake

Brady Lake

Esther Lake

ParadiseLakes

HomesteadCreek

Fall Creek

WhitneyCreek

East Fork HomestakeCreek

West Cross Creek

French Creek

Fancy Creek Missouri

Creek

Sopris Creek

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Table 2. Reaches and features that can carry geographic names (Notes for table: (1) Type: The table lists all reach and feature types defined for the NHD. Note that not all types have been collected in the initial release. (2) Geographic name: An "X" in the column indicates that a geographic name may be associated with reaches or features of this type. Note that geographic names have not been encoded for many reaches and features. (3) An asterisk (*) means that the feature or names are not in the initial release of the NHD. (4) Coastline reaches carry the name of the water feature that they bound. (5) For the State of Washington, waterbody reaches carry the names of the features types ice mass, lake/pond, reservoir, and swamp/marsh. Elsewhere, waterbody reaches carry the names of the features type lake/pond only.)

Type Geographic Name

Reach

Coastline X Transport X Shoreline < Not implemented > Waterbody X

Features

Anchorage * X * Area of Complex Channels Area To Be Submerged X Artificial Path On Reach Bay/Inlet * X * Bridge X Canal/Ditch On Reach Connector On Reach Crevasse Field * X * Dam/Weir X Estuary * X * Fish Ladder * Flume * X * Foreshore X * Fumarole X Gaging Station Gate Geyser X Hazard Zone Ice Mass X, On Reach in

Washington State Inundation Area X Lake/Pond On Reach Lock Chamber X Mud Pot X Nonearthen Shore Pipeline On Reach Playa X Post * Rapids X Reef X Reservoir X, On Reach in

Washington State Rock X *

Type Geographic Name

Sea/Ocean X Sink/Rise * X * Snag/Stump * Sounding Datum Line Special Use Zone Special Use Zone Limit Spillway Spring/Seep X Stream/River On Reach Submerged Stream X Swamp/Marsh X, On Reach in

Washington State Tunnel X Underpass Wall Wash X Water Intake/Outflow * Waterfall X Well X Wreck * X *

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Characteristics of domestic geographic names (Adapted from "Principles, policies, and procedures: domestic geographic names" (Orth and Payne, 1997, p. 3); "The national gazetteer of the United States of America: concise: (USGS, 1990, p. xi); and "Geographic Names Information System" (USGS, 1995, p. 7-8).)

Geographic names normally originate in and are influenced by spoken language. It is important to remember this fact because many people are concerned with written forms of names, including matters of spelling, capitalization, word form, and writing marks, that may have little to do with the way names are spoken. Geographic names in the United States most often reflect English, French, and Spanish naming traditions.

Most geographic names are binomial in that they have two parts, denoting specific and generic information. The generic part tells the kind of place, feature, or area to which the name refers and usually is a single topographic term, such as brook, creek, lake, spring, or river. The specific part uniquely identifies the particular place, feature, or area, and it may consist of one or more words. For example:

Geographic Name Specific Generic

Hudson River Hudson River

Lake Superior Superior Lake

Arroyo Chico Chico Arroyo

Rio Grande del Rancho Grande del Rancho Rio

Foot of the Mountain Run Foot of the Mountain Run

The binomial (two-part) form of specific and generic information is strong and in written usage often leads to combining words in the specific part of the name, such as Threemile Run and Redhill Gulch. The names of some features can be long, especially if the specific part is a prepositional phrase: Cliffs of the Seven Double Pillars, Foot of the Mountain Run, and Cañon del Rajadero de los Negros.

Some geographic names have a "false" generic that does not describe the feature correctly. For features of the type stream/river, common false generics are terms for depressions in the Earth's surface (such as "draw", "gully" or "gulch") that usually are not defined as applying to water flowing through the depression.

Some names have rare generic forms. Examples include colorful American names such as Bowl of Tears (lake), Butlers Toothpick (pinnacle rock), Titans Piazza (hill), and Devils Racepath (ridge). Among variations of the binomial form are one-word names that require a capitalized article: The Canal, The Lagoon, The Lakes, and La Laguna.

Entry conventions for geographic names The entries for geographic names include uppercase and lowercase alphabetic, numeric, and punctuation and other special characters. Most names are entered in the way they are commonly written; for example "Adams Creek" and "Green Lake". Exceptions include the following:

• Diacritical mark. A diacritical mark is a mark near or through a character or combination of characters. The mark indicates a phonetic value (a spoken sound) different from that given the unmarked or otherwise marked character. For example, the acute accent ("´") is the diacritical mark in río, and the tilde ("~") is the diacritical mark in cañada and cañoncito. If the entry for a name has a diacritical mark in the Geographic Names Information System, the mark is retained in the NHD9.

9 Geographic names are encoded using the "Latin 1" character set (International Organization for Standardization (ISO) standard 8859-1).

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26

• Generic part entered last. Sometimes the generic part of the name may be in the first position, followed by the specific part. For example, the name Rio Grande del Rancho begins with the generic "rio" (from the Spanish río for river). In most of these cases, the geographic name is entered with the position of the generic within the name reversed. For example, Rio Grande del Rancho becomes "Grande del Rancho, Rio". The generic, however, is not reversed if the specific part of the name contains an Arabic number.

• Initial definite articles entered last. Some geographic names start with a definite article (for example, "a" and "the" in English, and "el", "la", "los", and "las" in Spanish). Examples include El Capitan Wash and The Cat Hole. In many of these cases, the geographic name is entered so that the initial definite article is provided last. For example, El Capitan Wash becomes "Capitan Wash, El" and The Cat Hole becomes "Cat Hole, The".

Common identifier

The common identifier is a 10-digit integer value that uniquely identifies each feature or reach. Each value occurs only once throughout the Nation. Each feature and reach is assigned an identifier, stored in the data element named "COM_ID". Once assigned, the identifier is associated permanently with its feature or reach and should not be altered. When a feature or reach is deleted, its identifier is retired.

Common identifiers serve several purposes. They are the basis for relating features and reaches for several purposes, including the following:

• underpasses: the data elements "ABOVE_ID" and "BELOW_ID" store the common identifiers of the features above and below an underpass respectively.

• reach flow relations: the data elements "COM_ID_1" and "COM_ID_2" store the common identifiers of the reaches that participate in a flow relation.

• linear features and their transport and coastline reaches: the data element "RCH_COM_ID" stores the common identifier of the transport or coastline reach that underlies a linear feature.

• waterbodies and their waterbody reaches: the data element "RCH_COM_ID" stores the common identifier of the waterbody reach that underlies a waterbody.

• waterbodies and linear features they contain: the data element "WB_COM_ID" stores the common identifier of the waterbody that contains a linear feature.

• features and reaches, and their digital update units: the data element "COM_ID", when associated with the data element "DUU_ID", associates features and reaches with their digital update units.

Common identifiers are also the basis for tracking and sharing deletions, additions, and modifications of features, reaches, and relations.

Main points: • The common identifier is a 10-digit integer value that uniquely identifies each feature

or reach. • Common identifiers are the basis for relating features, underpasses, reaches, and

digital update units. • They are used to communicate and share corrections and updates among

organizations. • Never change a common identifier.

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Coordinates and related measures

Positions are encoded using a common coordinate referencing system. Other measurements, including lengths, areas, and selected surface elevations, also are encoded.

Horizontal coordinate referencing system The locations of points, lines, and the boundaries of areas are encoded using geographic (longitude-latitude) coordinates. The coordinates are encoded in decimal degrees, with west longitude and south latitude represented by negative values. The horizontal datum is the North American Datum of 1983 (NAD83).

Lengths and areas Lengths of linear features and areas of areal features are supplied for convenience in applications that require these measures. The coordinate data were projected from longitude-latitude coordinates to the Albers Equal Area projection, and lengths and areas were calculated and saved. The parameters for the projected coordinate systems are shown below:

Geographic Area Standard Parallels Central Meridian

Projection Origin

Conterminous United States 29° 30' North 45° 30' North 96° West 23° North

Alaska 55° North 65° North 154° West 50° North Hawaii 8° North 18° North 157° West 3° North Puerto Rico and the

Virgin Islands 8° North 18° North 66° West 3° North

The North American Datum of 1983 was specified for the projected coordinate system. The distance units of the projected system were meters.

Lengths are computed in meters and stored in data elements named "METERS". Areas are computed in square kilometers and stored in data elements named "SQ_KM".

Elevations of water surfaces The elevation of water surfaces where water pools is encoded for a few features. Water surface elevations may be encoded for features of the type area to be submerged, canal/ditch, inundation area, lake/pond, reservoir, and stream/river. Note that these elevations have not been entered for most features. The vertical datum for the conterminous United States and Alaska is the National Geodetic Vertical Datum of 1929. For Puerto Rico and the Virgin Islands, the datum is Local Mean Sea Level.

Elevations are recorded in meters and stored in data elements named "ELEV". Two encoded values have special meanings. A value of -9999 means that an elevation value will not be applied to the feature. A value of -9998 means that a value can be applied to the feature but has not been specified.

In addition to the elevation, the prevailing condition (for example, “average water elevation”) to which the elevation applies is provided. The condition is stored as a text string in data elements named "STAGE". Where the "ELEV" field has a value of -9999 or -9998, the "STAGE" field is blank.

Main points: • Latitude and longitude values are used for horizontal coordinates. • The horizontal datum is the North American Datum of 1983. • Lengths and areas of features and reaches, computed from a projected coordinate

system, are supplied for convenience. • Elevations of surfaces where water pools are supplied for some features.

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Data Quality

The NHD resulted from a multiyear effort to process and integrate datasets containing delineations and classification of features, reaches, hydrologic units, and geographic names. This section provides an overview of the production processes, sources of information used in these processes, and other statements related to data quality.

This section describes what is "generally true" about the initial release of the data for the conterminous United States, Hawaii, Puerto Rico, and the Virgin Islands. The NHD, even in its initial release, has variations in currentness, processes, and other characteristics. It is important to note that, even before the initial release of the data, collaborative efforts are under way to correct and improve the data. Completion of these efforts will contradict even these "generally true" statements for places where the improvements are made. It is important to review the metadata to understand the condition of data.

Conterminous United States (excluding the Pacific Northwest), Hawaii, Puerto Rico, and the Virgin Islands The discussion below applies to the conterminous United States (excluding the Pacific Northwest10), Hawaii, Puerto Rico, and the Virgin Islands.

10 In this data quality discussion, the "Pacific Northwest" includes the Pacific Northwest hydrologic region (region 17) and the Williamson, Sprague, and Upper Klamath Lake (Oregon) subbasins (18010201, -02, and –03, respectively). This area includes the State of Washington, most of Idaho and Oregon, and parts of western Montana and Wyoming.

Main points: • Because of variations within the NHD, it is difficult to make general statements about

data quality. Metadata associated with the digital update units are the best source of information for a place.

• Statements of data quality vary geographically. Different general statements are made for (1) most of the conterminous United States, Hawaii, Puerto Rico, and the Virgin Islands, (2) the Pacific Northwest, and (3) Alaska.

• For the conterminous United States (excluding the Pacific Northwest), Hawaii, Puerto Rico, and the Virgin Islands: • Lineage:

• Two main sources of information are digital line graph and Reach File Version 3 data. The most prominent map scale for sources is 1:100,000; larger scale data are being integrated as part of data improvement activities.

• Data processing efforts concentrated on creating features, adding artificial paths to areal features, integrating features with reaches and geographic names, orienting features and reaches in the direction of water flow, constructing flow relations, and validating the results of these efforts.

• Attribute accuracy assessments are based on those for the digital line graph data; geographic name entries are the same as those found in the Geographic Names Information System.

• Points, nodes, lines, and areas follow topological rules. • The data reflect the completeness of the source materials; these sources vary in

completeness. • The accuracy of horizontal and vertical positions is based on the assessments for

the source materials; these sources conformed to the National Map Accuracy Standard.

• Similar approaches are being used to develop data for the Pacific Northwest and Alaska. Specific statements will be provided when these data are released.

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Lineage Figure 16 illustrates the information sources and processes used to develop the data.

Figure 16. Information sources and processes used to create the NHD.

Information Sources Sources of information used to construct the initial release of the NHD include:

• Digital line graph 3 (DLG-3) data. These data, captured from USGS topographic maps and unpublished source materials, provide the delineations and classification of features (except for artificial paths in areal features and connectors). The data were organized in the DLG optional format, were tiled in quadrangles, were edge-matched, were from the DLG "hydrography" data category, and were in the Universal Transverse Mercator coordinate system, North American Datum of 1927 (NAD27). The scales of the map source materials used for the initial release of the data are shown below:

Geographic Area Map Scale of Source Information Conterminous United States and Hawaii 1:100,000 Puerto Rico 1:20,000 and 1:30,000 Virgin Islands 1:24,000

Efforts are under way to improve the NHD; most of these involve the collecting data for the conterminous United States from 1:24,000-scale maps, and digital images with positional accuracies commensurate with 1:12,000 and larger scales. Currentness varies by individual maps; see digital update units for more information.

• Reach File Version 3 (RF3) data. These data, developed by the USEPA, provide the starting point for reach delineation, reach codes, direction of water flow information, and positions of geographic names for the feature type stream/river. These data were developed on 1:100,000-scale DLG data for the conterminous United States and Hawaii as part of a previous project.

• Subbasin boundaries. These data, developed by different agencies, are used to assign features and reaches to subbasins.

Cataloging unit

boundaries

Reach File version 3

Convert to features

Digital Line Graph 3 files

Features

Build reaches

National Hydrography

Dataset

(Feb 1995)

(Dec 1996)

Geographicnames

(Mar 1999)

California and Arizona

updates

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Geographic Area Map Scale of Source Information Originator

Conterminous United States (States of Connecticut, Delaware, Florida, Kentucky, Minnesota, Montana, North Carolina, Oklahoma, South Carolina, and West Virginia)

1:100,000 State agencies in respective States

Conterminous United States (remainder) 1:250,000 USGS Puerto Rico 1:20,000 USEPA Hawaii and the Virgin Islands < No boundaries needed. Subbasins are based on islands. >

• Geographic names. • Final version. Extracted in March 1999 from the National Geographic Names Database of the

USGS's Geographic Names Information System, these data were the final source of geographic name entries.

• Earlier versions. Two earlier extracts from the Geographic Names Information System were used to assign names to reaches (December 1996 extract) and waterbodies (February 1995 extract).

• Features. These quadrangle-tiled data, containing features and characteristics, were created from the DLG-3 data.

• California and Arizona updates. These data from California and Arizona State agencies contain additional name assignments and minor spatial and attribute corrections to Reach File Version 3 data.

Processes

Convert DLG data to features This process converted DLG data to features and associated characteristics and converted the coordinate system to geographic (longitude-latitude) coordinates in the NAD83 in five steps:

1. The USGS's "Batch DLG-3 to DLG-F Conversion System" converted DLG-3 nodes, lines, areas, and associated attribute codes to temporary features and associated characteristics. Known conditions for which conversions could not be reliably made were flagged for later inspection. Only known conversion problems were flagged, and no additional steps were taken to detect or repair discrepancies in the original DLG-3 or the converted NHD.

2. A default value of a characteristic was added in cases where the description was incomplete.

3. All instances in which data were flagged were inspected and resolved interactively.

4. Feature delineation rules were applied to the temporary features in a batch process to create the final version of features.

5. Coordinate values were converted to geographic coordinates and to the NAD83 using the NADCON software version 2.1 (National Geodetic Survey, n.d.).

This process generated the "features" data.

Build reaches The basic steps for building reaches are as follows:

1. Convert RF3 to RF3" (RF3 double prime). This batch operation processed Reach File Version 3 to delete duplicate reaches, reassign reaches to the correct subbasin, validate geographic names assigned to reaches against data from the Geographic Names Information System (December 1996 extract), apply updates supplied by the States of California and Arizona, redelineate reaches on the basis of standards used for the NHD, and identify inflow/outflow points where transport reaches entered and exited waterbodies.

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2. Create artificial paths. Using waterbodies from the feature data and inflow/outflow points extracted from RF3", this process automatically generated the centerlines used to delineate artificial paths within waterbodies by using subroutines within the ARC/INFO11 GRID module.

3. Blind pass. This batch step conflated features and RF3" reaches and transferred reach information (reach code, reach date, name, stream level, and flow relationships) to the features. It also integrated the artificial paths generated in the previous step with the other features, built reaches on the artificial paths, and assigned geographic names (February 1995 extract) to waterbodies.

4. Quadrangle-based visual pass. During this interactive step, analysts ensured that the data developed in the previous batch processes conformed to reach delineation rules and that reaches were assigned to the appropriate subbasin. Batch procedures identified and developed a list of possible errors. (Errors not detected by the software may continue in the data.) Using the list, software presented each case to analysts to make appropriate edits to the data. Analysts recorded notes about repairs that could not be made and about other errors in the data. (These notes are encoded in the subbasin digital update units.)

5. Build superquads. After the quadrangle-based visual pass was complete, all quadrangles that cover all or part of each subbasin were paneled into a superquad. In this batch process, reaches that cross quad boundaries were corrected to conform to reach delineation rules.

6. Subbasin-based visual pass. As they did with the quadrangle-based visual pass, analysts ensured that reaches conformed to reach delineation rules. Batch procedures identified and developed a list of possible errors. (Errors not detected by the software may continue in the data.) Analysts examined each error and corrected the data. Analysts recorded notes about repairs that could not be made and about other errors in the data. (These notes are encoded in the subbasin digital update units.)

7. Central quality assurance/quality control. This step (1) confirmed that integrity checks were performed successfully during the visual pass activity, and (2) assessed statistics gathered during the earlier processes to determine if additional review of data was needed. A check of data from the subbasin-based visual pass was run in batch; any data that did not pass the procedure were reviewed interactively. If substantive changes were required, the data were reprocessed using procedures (as required) described in previous steps. The edited data then were rechecked using the central quality assurance/quality control process.

8. Data preparation and database load. This batch procedure performed final processing to the data emerging from the quality assurance/quality control step. Some of the activities included assigning the final reach codes, building waterbody reaches, adding final artificial paths in waterbodies, and implementing any recent changes in standards for the NHD. The spelling of geographic names was replaced using the March 1999 data extract from the Geographic Names Information System. After this, reaches, features, characteristics, geographic names, and relations were loaded into the database that holds the NHD.

9. Flow relation correction and validation. The flow relations were checked for consistency through a batch procedure, which generated a list of possible errors. Software presented possible errors to analysts, who corrected flow relations and, occasionally, the delineation of reaches. Changes were posted to the database.

10. Extract distribution copies of data. Data for a subbasin were extracted from the database and converted into an ARC/INFO workspace containing coverages and other files. Data available in the Spatial Data Transfer Standard format were developed from the workspaces. The workspaces and the Spatial Data Transfer Standard-formatted files were made available to the public.

Attribute accuracy The accuracy of the attributes of the DLG data is estimated to be 98.5 percent. One or more of the following methods was used to test attribute accuracy: 11 Identification of commercial products is for information only. No endorsement by the U.S. Government is implied or intended.

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• the source was manually compared with hardcopy plots. • a symbolized display was done of the DLG on an interactive computer graphics system. • selected attributes that could not be visually verified on plots or on screen were interactively queried

and verified.

In addition, software was used to validate feature types and characteristics against a master set of types and characteristics, to check that combinations of types and characteristics were valid, and to check that types and characteristics were valid for the delineation of the feature. Feature types, characteristics, and other attributes conform to the "Standards for National Hydrography Dataset" (USGS, 1999) as of the date they were loaded into the database.

The entry and identifier for geographic names match those in the Geographic Names Information System as of March 1999. The association of each name to reaches has not been methodically checked, and so a name may be applied to the wrong reaches. Anecdotal reviews indicate that 80 percent or more of the named reaches have the correct name.

Reaches were delineated with a batch procedure and were checked extensively during the "visual pass" steps of processing. According to automated quality assurance/quality control checks performed at various intervals during the processing, it is estimated that approximately 99 percent of the reaches are delineated according to standards.

Logical consistency Points, nodes, lines, and areas conform to topological rules. Lines intersect only at nodes, and all nodes anchor the ends of lines. Lines do not "overshoot" or "undershoot" other lines where they are supposed to meet. There are no duplicate lines within a dataset. Lines bound areas, and lines identify the areas to the left and right of the lines. Gaps and overlaps among areas do not exist. All areas close.

Completeness The completeness of the data reflects the content of the sources, which in the initial release of the NHD, are most often USGS topographic maps. Features found on the ground may have been eliminated or generalized on the source graphic because of scale and legibility constraints. In general, streams longer than 1mile (approximately 1.6 kilometers) were collected. Most streams that flow from a lake were collected regardless of their length. Only definite channels were collected, so not all swamp/marsh features have stream/rivers delineated through them. Lake/ponds having an area greater than 6 acres (approximately 2.4 hectares) were collected.

Note, however, that these general rules were applied unevenly among maps during compilation. Some map quadrangles have a much sparser pattern of hydrography than do adjoining maps, and these differences continue in the digital rendition of these features. Rectification of these differences is a priority for maintenance of the NHD.

Transport reaches are defined on almost all of the features of types stream/river, canal/ditch, pipeline, artificial path, and connector. Waterbody reaches are defined on the subset of lake/pond features that were identified as waterbodies during the development of Reach File Version 3.

Most attention in applying geographic names was given to transport reaches that follow stream/rivers and to waterbody reaches.

Near the international boundaries with Canada and Mexico, only the parts of features within the United States are delineated.

Detailed capture conditions are provided for every feature type in the "Standards for National Hydrography Dataset" (USGS, 1999), available online through http://mapping.usgs.gov/standards/.

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Horizontal positional accuracy Statements of horizontal positional accuracy are based on accuracy statements made for USGS topographic quadrangle maps. These maps were compiled to meet National Map Accuracy Standards. For horizontal accuracy, this standard is met if at least 90 percent of points tested are within 0.02 inch (at map scale) of their true positions.12 Additional offsets to positions may have been introduced where there are many features to improve the legibility of map symbols. In addition, the digitizing of maps is estimated to contain a horizontal positional error of less than or equal to 0.003-inch standard error (at map scale) in the two component directions relative to the source maps. Visual comparison between the map graphic (including digital scans of the graphic) and plots or digital displays of points, lines, and areas is used to assess the positional accuracy of digital data.

Linear features of the same type along the adjoining edges of datasets are aligned if they are within a 0.02-inch tolerance (at map scale). To align the features, the midpoint between the end of the corresponding features is computed, and the ends of features are moved to this point. Features outside the tolerance are not moved; instead, a feature of the type connector was added to join the features.

Vertical positional accuracy Statements of vertical positional accuracy for elevation of water surfaces are based on accuracy statements made for USGS topographic quadrangle maps. These maps were compiled to meet National Map Accuracy Standards. For vertical accuracy, this standard is met if at least 90 percent of well-defined points tested are within one-half contour interval of the correct value. Elevations of water surface printed on the published map meet this standard; the contour intervals of the maps vary. These elevations were transcribed into the digital data; the accuracy of this transcription was checked by visual comparison between the data and the map.

Pacific Northwest and Alaska Similar approaches are being used to develop data for the Pacific Northwest and Alaska. Specific statements will be provided when these data are released.

12 At 1:100,000 scale, 0.02 inch is approximately 167 feet (50.8 meters), and at 1:24,000 scale, 40 feet (approximately 12.2 meters).

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Glossary (The main sources for the definitions include Darcy and Boston (1988); Federal Geographic Data Committee (1994, 1997); Merriam-Webster (1998); Padmanabhan, Yoon, and Leipnik (1992); and Stamp and Clark (1979).)

ARC/INFO13 — Geographic information systems software developed by the Environmental Systems Research Institute of Redlands, California.

area — A generic term for a bounded, continuous, two-dimensional object that may or may not include its boundary. See line, node, and point.

cataloging unit — Currently known as subbasin.

characteristic — A distinguishing trait, quality, or property.

coastline — A line that follows the main outline of the land, including bays, but crosses rivers at their mouths. In the NHD, the outlines of selected coastal islands are included as part of the coastline. See shoreline.

coastline reach — A reach that represents a section of coastline.

common identifier — A 10-digit integer value that uniquely identifies each feature or reach in the NHD.

conflation — A process by which two sets of map data for the same region may be aligned and merged on the basis of matches of corresponding features portrayed in both sets.

confluence — Flowing together; the junction and union of two or more streams or moving fluids. In the NHD, this idea has been generalized to allow the feature types of artificial path, canal/ditch, connector, pipeline, and stream/river to meet at a confluence. See divergence.

conterminous — Having a common boundary; enclosed within a common boundary. conterminous United States — the lower 48 States and the District of Columbia.

contiguous — Touching along a boundary or at a point; touching or connected throughout in an unbroken sequence.

contour — An imaginary line on the ground, all points of which are at the same elevation.

contour interval — The difference in elevation between two adjacent contours.

coordinate reference system — A set of points, lines, and (or) surfaces and a set of rules that creates a reference frame whereby each point in a given surface can be identified uniquely by a set of numbers.

coordinates — Linear and (or) angular quantities that describe the location of a point in relation to a given coordinate reference system.

coverage — A digital version of a map forming the basic unit of vector data storage in ARC/INFO.

datum — Any quantity, or set of quantities, that may serve as a reference or basis for the calculation of other quantities. In relation to mapping and geographic information systems, datum usually refers to a set of quantities that serve as a reference for the calculation of positions. See horizontal datum, linear referencing system, and vertical datum.

13 Identification of commercial products is for information only. No endorsement by the U.S. Government is implied or intended.

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decimal degree — Representation of the measure of an angle using whole and decimal fractions of a degree. DECIMAL DEGREE = DEGREES + (MINUTES/60) + (SECONDS/3600). See latitude, longitude.

degree — An angular unit of measure equal to 1/360th of the circumference of a circle. See decimal degree, latitude, and longitude.

delineate — To mark the outline of; to describe, portray, or set forth with accuracy or in detail.

delineation — The act of delineating; something made by delineating.

diacritical mark — A mark near or through a character or combination of characters that indicates a phonetic value (a spoken sound) different from that given the unmarked or otherwise marked character.

digital update unit — A collection of one or more features and (or) reaches to which a set of metadata elements apply.

divergence — Flowing apart; the junction and separation of a stream or moving fluids into two or more paths. In the NHD, this idea has been generalized to allow the feature types of artificial path, canal/ditch, connector, pipeline, and stream/river to meet at a divergence. See confluence.

elevation — The vertical distance above or below a vertical datum to a point or object on the Earth's surface.

entity — Generally, something that has separate and distinct existence and objective or conceptual reality. In a database, an object of interest about which data can be collected.

feature — A defined entity and its representation. In the NHD, features include naturally occurring and constructed bodies of water, paths through which water flows, and related entities. These spatial phenomena are classified into defined feature types, are described by additional characteristics, and are delineated in standard ways.

feature code — A numeric value that encodes the type and values for a set of characteristics of a feature. This five-digit code has two parts: the first three digits encode the feature type; the last two digits encode values for a set of characteristics associated with the feature.

feature type — A member of a classification scheme for features. For example, in the NHD, features can be classified by types such as canal/ditch, lake/pond, and stream/river.

field — A single piece of information, the smallest unit normally manipulated by a database management system.

geocode — A location identifier. More specifically, a geocode is a data value assigned to a spatial object that encodes the location of the object (or a place along the object). A geocode can be used to associate other data with the object (or a place along the object). The term geocode also denotes the process of assigning a location identifier to an object. In the NHD, reach codes provide a starting point for geocoding observations to or along hydrographic features.

geographic name — The proper name, specific term, or expression by which a particular geographical entity is, or was, known.

head — The source of a stream.

horizontal datum — A set of constants to which horizontal coordinates are referred; a reference for position.

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hydrography — (1) The science comprising the description, study, and mapping of the waters of the Earth's surface (the seas, lakes, rivers, and so on), including their forms and physical features. (2) The subject matter of this science, the hydrographic features of the globe or part of it; the distribution of water on the Earth's surface.

hydrologic unit — A member of the hierarchical system for identifying and subdividing river-basin units of the United States. Hydrologic units are used for the collection and organization of hydrologic data. The levels of the hierarchy, listed in order of largest to smallest in area, are region, subregion, accounting unit, and subbasin. For example:

region — New England region: the drainage within the United States that ultimately discharges into: (a) the Bay of Fundy, (b) the Atlantic Ocean within and between the States of Maine and Connecticut, (c) Long Island Sound north of the New York-Connecticut state line, and (d) the Riviere St. Francois, a tributary of the St. Lawrence River.

subregion — Connecticut: the Connecticut River basin. accounting unit — Lower Connecticut: the Connecticut River basin below Vernon Dam. subbasin — Chicopee.

Each hydrologic unit is identified uniquely with a hydrologic unit code.

hydrologic unit code — A hierarchical, numeric code that uniquely identifies hydrologic units. The first two digits identify the region, the first four digits identify subregions, the first six digits identify accounting units, and the full eight digits identify subbasin. For example, from the example provided with the definition of hydrologic unit, the hydrologic unit codes are:

01 — the region (New England) 0108 — the subregion (Connecticut) 010802 — the accounting unit (Lower Connecticut) 01080204 — the subbasin (Chicopee)

Zeroes in the two-digit accounting unit field indicate that the accounting unit and the subregion are the same. Zeroes in the two-digit subbasin field indicate that the subbasin and the accounting unit are the same.

latitude — Distance north or south of the Equator, measured as an angle with the center of the Earth. In the NHD, latitude values are encoded in decimal degrees. See longitude.

level path — A sequence of transport reaches that trace the main stem for a given flow of water.

line — A generic term for a one-dimensional object having a length and direction. See area, node, and point.

linear referencing system — A set of datums, networks, and linear referencing methods, whereby each point along a network can be identified uniquely by specifying the direction and distance from any known point on the network.

longitude — Distance east or west on the Earth's surface, measured by the angle that the meridian of a particular place makes with the Prime (Greenwich) Meridian. In the NHD, longitude values are encoded in decimal degrees. See latitude.

lookup table — A table that provides the ability to use a known value to locate an unknown value. In the NHD, the feature code links features to the textual descriptions of their associated characteristics.

map scale — The relationship between a distance on a map, chart, or photograph and the corresponding distance on the Earth. For map scales commonly associated with the NHD, see the following table:

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Scale 1 inch equals 1 centimeter equals 1:100,000 1.6 miles (approximate) 1 kilometer 1:63,360 1 mile 0.63 kilometer (approximate) 1:30,000 2,500 feet 0.30 kilometer 1:24,000 2,000 feet 0.24 kilometer 1:20,000 1,667 feet (approximate) 0.20 kilometer 1:12,000 1,000 feet 0.12 kilometer

metadata — Data about data; data that describe the content, quality, condition, and other characteristics of data.

mouth — The place where a stream enters a larger body of water.

node — A zero-dimensional object that is the topological junction of two or more lines, or an end point of a line. See area, line, and point.

point — A zero-dimensional object that specifies geometric location. See area, line, and node.

quadrangle — A four-sided area, bounded by parallels of latitude and meridians of longitude, used as an areal unit in mapping.

reach — A continuous unbroken stretch or expanse of surface water. In the NHD, this idea has been expanded to define reach as a significant segment of surface water that has similar hydrologic characteristics. Reaches have standard types and delineations. See coastline reach, shoreline reach, transport reach, and waterbody reach.

reach code — A numeric code that uniquely identifies a reach. This 14-digit code has 2 parts: the first 8 digits are the hydrologic unit code for the subbasin in which the reach is located; the last 6 digits are a sequentially ordered, arbitrarily assigned number.

relation — In a database, a named association among sets of entities.

shoreline — The line where water and land meet. In the NHD, shoreline applies to inland waters only. See coastline.

shoreline reach — A reach that represents all or part of a shoreline. This type of reach has not been implemented.

stage — The elevation of the surface of a body of water measured relative to a vertical datum.

subbasin — The fourth level of subdivision of hydrologic units. A subbasin represents the geographic area of part or all of a surface drainage basin, a combination of drainage basins, or a distinct hydrologic feature. Subbasins are uniquely identified with an eight-digit hydrologic unit code.

terminus (plural termini) — A finishing point; a part that forms the end.

topology — A branch of geometrical mathematics concerned with order, contiguity, and relative position rather than actual linear dimensions. Topology is used to establish and describe spatial relationships among features.

transport reach — A reach that represents the pathway for the flow of water through a drainage network.

traverse — To move or pass along. traversal — An instance of traversing.

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tributary — A stream or river that flows into a larger one. In the NHD, this idea has been generalized to allow transport reaches with underlying feature types of artificial path, canal/ditch, connector, pipeline, and stream/river to have or be tributaries.

vertical datum — A set of constants specifying a coordinate system to which elevations are referred.

waterbody — A hydrographic feature that is delineated using areas.

waterbody reach — A reach that represents a waterbody. (In contrast, a transport reach represents the flows of water through such areas.)

workspace — A directory containing geographic datasets for use with ARC/INFO.

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References Allord, G.J., 1992, 1 to 2,000,000 hydrologic unit map of the conterminous United States (digital dataset):

Reston, VA, U.S. Geological Survey, http://water.usgs.gov/lookup/get?huc.

Darcy, L., and Boston, L., comps., 1988, Webster's new world dictionary of computer terms (3d ed.): New York, Simon & Schuster, Inc.

Federal Geographic Data Committee, 1994, Content standards for digital geospatial metadata: Washington, Federal Geographic Data Committee, 66 p. [http://www.fgdc.gov/metadata/metadata.html]

Federal Geographic Data Committee, 1997, Framework introduction and guide: Washington, Federal Geographic Data Committee, 105 p. [http://www.fgdc.gov/framework/frameworkintroguide/]

Merriam-Webster, 1998, WWWebster: Merriam-Webster's collegiate dictionary (10th ed.) [Online]: Springfield, MA, Merriam-Webster, Inc., http://www.m-w.com/netdict.htm.

National Geodetic Survey, [n.d.], NADCON version 2.1 [Software]: Silver Spring, MD, National Geodetic Survey. [Information about the current version of the software is available through http://www.ngs.noaa.gov/PC_PROD/pc_prod.html.]

Orth, D., and Payne, R., 1997, Principles, policies, and procedures: domestic geographic names: Reston, VA, U.S. Geological Survey, 51 p. [http://mapping.usgs.gov/www/gnis/bgn.html]

Padmanabhan, G., Yoon, J., and Leipnik, M., comps., 1992, A glossary of GIS terminology: National Center for Geographic Information and Analysis Technical Report 92-13, 79 p.

Seaber, P., Kapinos, F.P., and Knapp, G., 1987, Hydrologic unit maps: U.S. Geological Survey Water-Supply Paper 2294, 63 p., 1 pl. (Reprinted 1994.)

Stamp, L.D., and Clark, A., eds., 1979 [1981], A glossary of geographical terms (3d ed.): New York, Longman, 571 p.

U.S. Environmental Protection Agency, 1994, The U.S. EPA reach file version 3.0 Alpha release (RF3-Alpha) technical reference [Online], http://www.epa.gov/OWOW/NPS/rf/techref.html

U.S. Geological Survey, 1990, The national gazetteer of the United States of America: concise: U.S. Geological Survey Professional Paper 1200-US, 526 p.

U.S. Geological Survey, 1995, Geographic names information system: U.S. Geological Survey data users guide 6, 29 p. [http://mapping.usgs.gov/www/ti/GNIS/gnis_users_guide_toc.html]

U.S. Geological Survey, 1998, Hydrologic unit maps [Online], http://water.usgs.gov/lookup/get?huc.

U.S. Geological Survey, 1999, Standards for National Hydrography Dataset: Reston, VA, U.S. Geological Survey, http://mapping.usgs.gov/standards/

U.S. Geological Survey, 1999, National Atlas, Hydrologic unit boundaries [Online], http://www.nationalatlas.gov/hucsm.htm.

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Appendix A. Overview of features, reaches, and related items.

Table 1A. Feature types and related items Notes on table: (1) Feature type: The table lists all feature types and characteristics defined for the NHD. Note that not all feature types have been collected. (2) Geo(graphic) name: An X in the column indicates that a geographic name may be associated with features of this type. Note that geographic names have not been encoded for many features. (3) For the State of Washington, waterbody reaches carry the geographic names of lake/pond, reservoir, ice mass, and swamp/marsh. (4) Surface elevation: An X in the column indicates that the elevation of the surface of the water may be associated with the features of this feature type. Note that the surface elevations have not been encoded for most features. (5) An asterisk (*) means that the item is not in the initial release of the NHD.

Feature Type Characteristics Geo Name

Possible Delineation

Surface Elevation

Can relate to Underpass

Anchorage * Anchorage Type * X * Point *, Area * Area of Complex

Channels Area

Area To Be Submerged

X Area X

Artificial Path On Reach Line X Bay/Inlet * X * Area * Bridge X Line, Area Canal/Ditch Canal/Ditch Type On Reach Line, Area X X Connector On Reach Line Crevasse Field * X * Area * Dam/Weir Construction Material

Operational Status X Line, Area

Estuary * X * Area * Fish Ladder * Point *, Line *,

Area *

Flume * X * Line *, Area * Foreshore X * Area Fumarole X Point Gaging Station Point Gate Gate Type Point, Line Geyser X Point Hazard Zone Hazard Zone Category Area Ice Mass Ice Mass Category

Glaciation Category * X, On

Reach in Washington

State

Area

Inundation Area Inundation Control Status Inundation Area Type X Area X

Lake/Pond Hydrographic Category Water Characteristics On Reach Area X

Lock Chamber X Point, Line, Area

Mud Pot X Point Nonearthen Shore Line Pipeline Product

Pipeline Type Relationship to Surface

On Reach Line

Playa X Area

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Feature Type Characteristics Geo Name

Possible Delineation

Surface Elevation

Can relate to Underpass

Post * Post Type * Point * Rapids X Point, Line,

Area

Reef X Line Reservoir Construction Material

Cover Status Hydrographic Category Reservoir Type

X, On Reach in

Washington State

Point *, Area X

Rock Relationship to Surface X * Point Sea/Ocean Sea/Ocean Category X Area Sink/Rise * X * Point *, Line * Snag/Stump * Snag/Stump Type *

Relationship to Surface * Point *

Sounding Datum Line Positional Accuracy Line Special Use Zone Operational Status

Special Use Zone Type Area

Special Use Zone Limit

Positional Accuracy Line

Spillway Area Spring/Seep Water Characteristics X Point Stream/River Hydrographic Category

Positional Accuracy On Reach Line, Area X X

Submerged Stream X Area Swamp/Marsh X, On

Reach in Washington

State

Area

Tunnel X Line Underpass Point Not applicable Wall Wall Type Line Wash X Area Water Intake/Outflow * Water Intake/Outflow Type

* Point *, Area *

Waterfall X Point, Line Well Flow Status

Product Water Characteristics

X Point

Wreck * Abovewater Portion * Relationship to Surface * X * Point *

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Table 2A. Reaches and related items Notes on table: (1) Geo(graphic) name: An X in the column indicates that a geographic name may be associated with the reaches of this type. Note that geographic names have not been encoded for many reaches, especially those that follow the feature types canal/ditch or pipeline. Coastline reaches carry the name of the water feature that they bound. (2) Stream level: For coastline reaches, stream level is assigned the value of "-9998", which means that no value has been specified.

Reach Type Related Items Geo Name Delineation Features Types That

Provide Delineation Participates in the

Reach Flow RelationCoastline Reach Code


Stream Level

X Line Artificial Path X

Shoreline This reach type has not been implemented. Transport Reach Code


Stream Level

X Line Artificial Path, Canal/Ditch, Connector, Pipeline,

Stream/River

X

Waterbody Reach Code Reach Code

Assignment Date

X Area Only Lake/Pond for the initial release in most places.

Ice Mass, Lake/Pond, Reservoir, and Swamp/Marsh in the initial release for Washington State.

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Appendix B. Feature code and description field structures and definitions

Feature code structure The general format of the feature code is as follows:

FFFCC, in which

FFF is the three-digit code for a feature type.

CC is the two-digit code for a combination of values for characteristics associated with a feature type. If only the feature type is identified, these last two digits are assigned the value "0".

For example, the feature type "Dam/Weir" has the three-digit identifier "343". This number serves as the first three digits of the feature code. A code of "34300" is used if no characteristics are encoded for the feature.

A feature classified as "Dam/Weir" also may have the characteristic of "Construction Material" with the values of "earthen" and "nonearthen", and the characteristic of "Operational Status" with the values of "operational" and "under construction". The codes used to encode the combinations of values are shown below:

Operational Status Dam/Weir (Feature Type Number 343) operational under construction

Earthen 34301 34302 Construction Materials Nonearthen 34303 34304

Table 1B and Table 2B list all feature codes and their meanings.

Description field structure The description field uses a single character string to encode one or more characteristics and values for each feature code. The general format of the character string is shown below:

characteristic|value, in which

"characteristic" is the name of the characteristic.

The vertical line character ("|") separates the characteristic and value.

"value" is the value of the characteristic for the feature code.

When two or more characteristics are encoded by a feature code, these characteristic-value pairs are separated by the semicolon and space characters ("; "). If no characteristics are encoded (that is, if the code ends in "00"), the text "Feature type only: no attributes" is encoded.

For example, from the "Dam/Weir" characteristics and values described above, the following character strings are encoded for the feature codes:

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Feature Code Description

34300 Feature type only: no attributes 34301 Construction Material|earthen; Operational Status|operational 34302 Construction Material|earthen; Operational Status|under construction 34303 Construction Material|nonearthen; Operational Status|operational 34304 Construction Material|nonearthen; Operational Status|under construction

Table 1B and Table 2B list all character strings.

The document "Standards for National Hydrography Dataset" (USGS, 1999) contains the names and definitions of all feature types, characteristics, and values. The document is available online through http://mapping.usgs.gov/standards/.

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Table 1B. Feature code definitions listed numerically by feature code (FCODE) (Note that many of the listed codes were not required in the initial release of the NHD. Appendix A lists feature types and characteristics that were not encoded.)

FCODE Feature Type Description 30200 ANCHORAGE feature type only: no attributes

30201 ANCHORAGE Anchorage Type|explosives isolation

30202 ANCHORAGE Anchorage Type|quarantine

30203 ANCHORAGE Anchorage Type|seaplane

30700 AREA TO BE SUBMERGED

feature type only: no attributes

31200 BAY/INLET feature type only: no attributes

31800 BRIDGE feature type only: no attributes

33400 CONNECTOR feature type only: no attributes

33600 CANAL/DITCH feature type only: no attributes

33601 CANAL/DITCH Canal/Ditch Type|aqueduct

33602 CANAL/DITCH Canal/Ditch Type|unspecified

34000 CREVASSE FIELD feature type only: no attributes

34300 DAM/WEIR feature type only: no attributes

34301 DAM/WEIR Construction Material|earthen; Operational Status|operational

34302 DAM/WEIR Construction Material|earthen; Operational Status|under construction

34303 DAM/WEIR Construction Material|nonearthen; Operational Status|operational

34304 DAM/WEIR Construction Material|nonearthen; Operational Status|under construction

36000 FISH LADDER feature type only: no attributes

36100 PLAYA feature type only: no attributes

36200 FLUME feature type only: no attributes

36400 FORESHORE feature type only: no attributes

36600 FUMAROLE feature type only: no attributes

36700 GAGING STATION feature type only: no attributes

36900 GATE feature type only: no attributes

36901 GATE Gate Type|drydock

36902 GATE Gate Type|floodgate

36903 GATE Gate Type|lock

36904 GATE Gate Type|tidegate

36905 GATE Gate Type|unspecified

37000 GEYSER feature type only: no attributes

37300 HAZARD ZONE feature type only: no attributes

37301 HAZARD ZONE Hazard Zone Category|unspecified

37302 HAZARD ZONE Hazard Zone Category|crib area

37303 HAZARD ZONE Hazard Zone Category|foul ground

37304 HAZARD ZONE Hazard Zone Category|mine danger area

37305 HAZARD ZONE Hazard Zone Category|piling area

37306 HAZARD ZONE Hazard Zone Category|platform area

37307 HAZARD ZONE Hazard Zone Category|reef area

37308 HAZARD ZONE Hazard Zone Category|rock area

37309 HAZARD ZONE Hazard Zone Category|shoal

37310 HAZARD ZONE Hazard Zone Category|snag/stump area

37311 HAZARD ZONE Hazard Zone Category|well area

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FCODE Feature Type Description 37312 HAZARD ZONE Hazard Zone Category|wreckage

37800 ICE MASS feature type only: no attributes

37801 ICE MASS Ice Mass Category|alpine glacier

37802 ICE MASS Ice Mass Category|continental glacier; Glaciation Category|ice shelf

37803 ICE MASS Ice Mass Category|continental glacier; Glaciation Category|inland ice sheet

37804 ICE MASS Ice Mass Category|pack ice

37805 ICE MASS Ice Mass Category|snowfield

37806 ICE MASS Ice Mass Category|unspecified

39000 LAKE/POND feature type only: no attributes

39001 LAKE/POND Hydrographic Category|intermittent; Water Characteristics|salt

39002 LAKE/POND Hydrographic Category|intermittent; Water Characteristics|unspecified

39003 LAKE/POND Hydrographic Category|perennial; Water Characteristics|salt

39004 LAKE/POND Hydrographic Category|perennial; Water Characteristics|unspecified

39800 LOCK CHAMBER feature type only: no attributes

40300 INUNDATION AREA feature type only: no attributes

40301 INUNDATION AREA Inundation Control Status|controlled; Inundation Area Type|debris basin

40302 INUNDATION AREA Inundation Control Status|controlled; Inundation Area Type|dewatering area

40303 INUNDATION AREA Inundation Control Status|controlled; Inundation Area Type|duck pond

40304 INUNDATION AREA Inundation Control Status|controlled; Inundation Area Type|general case

40305 INUNDATION AREA Inundation Control Status|controlled; Inundation Area Type|percolation basin

40306 INUNDATION AREA Inundation Control Status|controlled; Inundation Area Type|retarding basin

40307 INUNDATION AREA Inundation Control Status|not controlled

40900 MUD POT feature type only: no attributes

41100 NONEARTHERN SHORE


42200 POST feature type only: no attributes

42201 POST Post Type|dolphin

42202 POST Post Type|piling

42800 PIPELINE feature type only: no attributes

42801 PIPELINE Product|water; Pipeline Type|aqueduct; Relationship to Surface|at or near

42802 PIPELINE Product|water; Pipeline Type|aqueduct; Relationship to Surface|elevated

42803 PIPELINE Product|water; Pipeline Type|aqueduct; Relationship to Surface|underground

42804 PIPELINE Product|water; Pipeline Type|aqueduct; Relationship to Surface|underwater

42805 PIPELINE Product|water; Pipeline Type|general case; Relationship to Surface|at or near

42806 PIPELINE Product|water; Pipeline Type|general case; Relationship to Surface|elevated

42807 PIPELINE Product|water; Pipeline Type|general case; Relationship to Surface|underground

42808 PIPELINE Product|water; Pipeline Type|general case; Relationship to Surface|underwater

42809 PIPELINE Product|water; Pipeline Type|penstock; Relationship to Surface|at or near

42810 PIPELINE Product|water; Pipeline Type|penstock; Relationship to Surface|elevated

42811 PIPELINE Product|water; Pipeline Type|penstock; Relationship to Surface|underground

42812 PIPELINE Product|water; Pipeline Type|penstock; Relationship to Surface|underwater

42813 PIPELINE Product|water; Pipeline Type|siphon; Relationship to Surface|unspecified

43100 RAPIDS feature type only: no attributes

43400 REEF feature type only: no attributes

43600 RESERVOIR feature type only: no attributes

43601 RESERVOIR Reservoir Type|aquaculture; Construction Material|unspecified

43602 RESERVOIR Reservoir Type|decorative pool; Construction Material|nonearthen

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FCODE Feature Type Description 43603 RESERVOIR Reservoir Type|decorative pool; Construction Material|unspecified

43604 RESERVOIR Reservoir Type|disposal-tailings pond; Construction Material|earthen

43605 RESERVOIR Reservoir Type|disposal-tailings pond; Construction Material|unspecified

43606 RESERVOIR Reservoir Type|disposal-unspecified; Construction Material|unspecified

43607 RESERVOIR Reservoir Type|evaporator; Construction Material|unspecified

43608 RESERVOIR Reservoir Type|swimming pool; Construction Material|nonearthen

43609 RESERVOIR Reservoir Type|treatment-cooling pond; Construction Material|unspecified

43610 RESERVOIR Reservoir Type|treatment-filtration pond; Construction Material|unspecified

43611 RESERVOIR Reservoir Type|treatment-settling pond; Construction Material|unspecified

43612 RESERVOIR Reservoir Type|treatment-sewage treatment pond; Construction Material|unspecified

43613 RESERVOIR Reservoir Type|water storage; Construction Material|nonearthen; Cover Status|covered; Hydrographic Category|unspecified

43614 RESERVOIR Reservoir Type|water storage; Construction Material|earthen; Cover status|not covered; Hydrographic Category|intermittent

43615 RESERVOIR Reservoir Type|water storage; Construction Material|earthen; Cover status|not covered; Hydrographic Category|perennial

43616 RESERVOIR Reservoir Type|water storage; Construction Material|nonearthen; Cover status|not covered; Hydrographic Category|unspecified

43617 RESERVOIR Reservoir Type|water storage; Construction Material|unspecified; Cover status|unspecified; Hydrographic Category|unspecified

43618 RESERVOIR Reservoir Type|unspecified; Construction Material|earthen

43619 RESERVOIR Reservoir Type|unspecified; Construction Material|nonearthen

43620 RESERVOIR Reservoir Type|unspecified; Construction Material|unspecified

44100 ROCK feature type only: no attributes

44101 ROCK Relationship to Surface|abovewater

44102 ROCK Relationship to Surface|underwater

44500 SEA/OCEAN feature type only: no attributes

44501 SEA/OCEAN Sea/Ocean Category|reef pool

44502 SEA/OCEAN Sea/Ocean Category|unspecified

45000 SINK/RISE feature type only: no attributes

45200 SNAG/STUMP feature type only: no attributes

45201 SNAG/STUMP Snag/Stump Type|snag; Relationship to Surface|abovewater

45202 SNAG/STUMP Snag/Stump Type|snag; Relationship to Surface|underwater

45203 SNAG/STUMP Snag/Stump Type|stump; Relationship to Surface|abovewater

45204 SNAG/STUMP Snag/Stump Type|stump; Relationship to Surface|underwater

45400 SPECIAL USE ZONE feature type only: no attributes

45401 SPECIAL USE ZONE Special Use Zone Type|dump site; Operational Status|operational

45402 SPECIAL USE ZONE Special Use Zone Type|dump site; Operational Status|abandoned

45403 SPECIAL USE ZONE Special Use Zone Type|spoil area; Operational Status|operational

45404 SPECIAL USE ZONE Special Use Zone Type|spoil area; Operational Status|abandoned

45500 SPILLWAY feature type only: no attributes

45800 SPRING/SEEP feature type only: no attributes

45801 SPRING/SEEP Water Characteristics|unspecified

45802 SPRING/SEEP Water Characteristics|alkaline

45803 SPRING/SEEP Water Characteristics|hot

45804 SPRING/SEEP Water Characteristics|sulphur

46000 STREAM/RIVER feature type only: no attributes

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FCODE Feature Type Description 46001 STREAM/RIVER Hydrographic Category|intermittent; Positional Accuracy|definite

46002 STREAM/RIVER Hydrographic Category|intermittent; Positional Accuracy|indefinite

46003 STREAM/RIVER Hydrographic Category|intermittent; Positional Accuracy|not applicable

46004 STREAM/RIVER Hydrographic Category|perennial; Positional Accuracy|definite

46005 STREAM/RIVER Hydrographic Category|perennial; Positional Accuracy|indefinite

46006 STREAM/RIVER Hydrographic Category|perennial; Positional Accuracy|not applicable

46100 SUBMERGED STREAM feature type only: no attributes

46600 SWAMP/MARSH feature type only: no attributes

47800 TUNNEL feature type only: no attributes

48100 UNDERPASS feature type only: no attributes

48300 WALL feature type only: no attributes

48301 WALL Wall Type|general case

48302 WALL Wall Type|sea

48400 WASH feature type only: no attributes

48500 WATER INTAKE/OUTFLOW



Water Intake/Outflow Type|intake


Water Intake/Outflow Type|outflow

48700 WATERFALL feature type only: no attributes

48800 WELL feature type only: no attributes

48801 WELL Product|heat

48802 WELL Product|water; Flow Status|flowing; Water Characteristics|unspecified

48803 WELL Product|water; Flow Status|unspecified; Water Characteristics|alkaline

48804 WELL Product|water; Flow Status|unspecified; Water Characteristics|hot

48805 WELL Product|water; Flow Status|unspecified; Water Characteristics|sulphur

48806 WELL Product|water; Flow Status|unspecified; Water Characteristics|unspecified

49200 WRECK feature type only: no attributes

49201 WRECK Relationship to Surface|abovewater; Abovewater Portion|hull and/or superstructure

49202 WRECK Relationship to Surface|abovewater; Abovewater Portion|mast and/or funnel

49203 WRECK Relationship to Surface|underwater

49300 ESTUARY feature type only: no attributes

50300 SOUNDING DATUM LINE



Positional Accuracy|approximate


Positional Accuracy|definite

53300 SPECIAL USE ZONE LIMIT



Positional Accuracy|definite


Positional Accuracy|indefinite

53700 AREA OF COMPLEX CHANNELS


55800 ARTIFICIAL PATH feature type only: no attributes

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51

Table 2B. Feature code definitions listed alphabetically by feature type (Note that many of the listed codes were not required in the NHD. Appendix A lists feature types and characteristics that were not encoded.)

Feature Type FCODE Description ANCHORAGE 30200 feature type only: no attributes

ANCHORAGE 30201 Anchorage Type|explosives isolation

ANCHORAGE 30202 Anchorage Type|quarantine

ANCHORAGE 30203 Anchorage Type|seaplane

AREA OF COMPLEX CHANNELS

53700 feature type only: no attributes

AREA TO BE SUBMERGED


ARTIFICIAL PATH 55800 feature type only: no attributes

BAY/INLET 31200 feature type only: no attributes

BRIDGE 31800 feature type only: no attributes

CANAL/DITCH 33600 feature type only: no attributes

CANAL/DITCH 33601 Canal/Ditch Type|aqueduct

CANAL/DITCH 33602 Canal/Ditch Type|unspecified

CONNECTOR 33400 feature type only: no attributes

CREVASSE FIELD 34000 feature type only: no attributes

DAM/WEIR 34300 feature type only: no attributes

DAM/WEIR 34301 Construction Material|earthen; Operational Status|operational

DAM/WEIR 34302 Construction Material|earthen; Operational Status|under construction

DAM/WEIR 34303 Construction Material|nonearthen; Operational Status|operational

DAM/WEIR 34304 Construction Material|nonearthen; Operational Status|under construction

ESTUARY 49300 feature type only: no attributes

FISH LADDER 36000 feature type only: no attributes

FLUME 36200 feature type only: no attributes

FORESHORE 36400 feature type only: no attributes

FUMAROLE 36600 feature type only: no attributes

GAGING STATION 36700 feature type only: no attributes

GATE 36900 feature type only: no attributes

GATE 36901 Gate Type|drydock

GATE 36902 Gate Type|floodgate

GATE 36903 Gate Type|lock

GATE 36904 Gate Type|tidegate

GATE 36905 Gate Type|unspecified

GEYSER 37000 feature type only: no attributes

HAZARD ZONE 37300 feature type only: no attributes

HAZARD ZONE 37301 Hazard Zone Category|unspecified

HAZARD ZONE 37302 Hazard Zone Category|crib area

HAZARD ZONE 37303 Hazard Zone Category|foul ground

HAZARD ZONE 37304 Hazard Zone Category|mine danger area

HAZARD ZONE 37305 Hazard Zone Category|piling area

HAZARD ZONE 37306 Hazard Zone Category|platform area

HAZARD ZONE 37307 Hazard Zone Category|reef area

HAZARD ZONE 37308 Hazard Zone Category|rock area

HAZARD ZONE 37309 Hazard Zone Category|shoal

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Feature Type FCODE Description HAZARD ZONE 37310 Hazard Zone Category|snag/stump area

HAZARD ZONE 37311 Hazard Zone Category|well area

HAZARD ZONE 37312 Hazard Zone Category|wreckage

ICE MASS 37800 feature type only: no attributes

ICE MASS 37801 Ice Mass Category|alpine glacier

ICE MASS 37802 Ice Mass Category|continental glacier; Glaciation Category|ice shelf

ICE MASS 37803 Ice Mass Category|continental glacier; Glaciation Category|inland ice sheet

ICE MASS 37804 Ice Mass Category|pack ice

ICE MASS 37805 Ice Mass Category|snowfield

ICE MASS 37806 Ice Mass Category|unspecified

INUNDATION AREA 40300 feature type only: no attributes

INUNDATION AREA 40301 Inundation Control Status|controlled; Inundation Area Type|debris basin

INUNDATION AREA 40302 Inundation Control Status|controlled; Inundation Area Type|dewatering area

INUNDATION AREA 40303 Inundation Control Status|controlled; Inundation Area Type|duck pond

INUNDATION AREA 40304 Inundation Control Status|controlled; Inundation Area Type|general case

INUNDATION AREA 40305 Inundation Control Status|controlled; Inundation Area Type|percolation basin

INUNDATION AREA 40306 Inundation Control Status|controlled; Inundation Area Type|retarding basin

INUNDATION AREA 40307 Inundation Control Status|not controlled

LAKE/POND 39000 feature type only: no attributes

LAKE/POND 39001 Hydrographic Category|intermittent; Water Characteristics|salt

LAKE/POND 39002 Hydrographic Category|intermittent; Water Characteristics|unspecified

LAKE/POND 39003 Hydrographic Category|perennial; Water Characteristics|salt

LAKE/POND 39004 Hydrographic Category|perennial; Water Characteristics|unspecified

LOCK CHAMBER 39800 feature type only: no attributes

MUD POT 40900 feature type only: no attributes

NONEARTHERN SHORE


PIPELINE 42800 feature type only: no attributes

PIPELINE 42801 Product|water; Pipeline Type|aqueduct; Relationship to Surface|at or near

PIPELINE 42802 Product|water; Pipeline Type|aqueduct; Relationship to Surface|elevated

PIPELINE 42803 Product|water; Pipeline Type|aqueduct; Relationship to Surface|underground

PIPELINE 42804 Product|water; Pipeline Type|aqueduct; Relationship to Surface|underwater

PIPELINE 42805 Product|water; Pipeline Type|general case; Relationship to Surface|at or near

PIPELINE 42806 Product|water; Pipeline Type|general case; Relationship to Surface|elevated

PIPELINE 42807 Product|water; Pipeline Type|general case; Relationship to Surface|underground

PIPELINE 42808 Product|water; Pipeline Type|general case; Relationship to Surface|underwater

PIPELINE 42809 Product|water; Pipeline Type|penstock; Relationship to Surface|at or near

PIPELINE 42810 Product|water; Pipeline Type|penstock; Relationship to Surface|elevated

PIPELINE 42811 Product|water; Pipeline Type|penstock; Relationship to Surface|underground

PIPELINE 42812 Product|water; Pipeline Type|penstock; Relationship to Surface|underwater

PIPELINE 42813 Product|water; Pipeline Type|siphon; Relationship to Surface|unspecified

PLAYA 36100 feature type only: no attributes

POST 42200 feature type only: no attributes

POST 42201 Post Type|dolphin

POST 42202 Post Type|piling

RAPIDS 43100 feature type only: no attributes

REEF 43400 feature type only: no attributes

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Feature Type FCODE Description RESERVOIR 43600 feature type only: no attributes

RESERVOIR 43601 Reservoir Type|aquaculture; Construction Material|unspecified

RESERVOIR 43602 Reservoir Type|decorative pool; Construction Material|nonearthen

RESERVOIR 43603 Reservoir Type|decorative pool; Construction Material|unspecified

RESERVOIR 43604 Reservoir Type|disposal-tailings pond; Construction Material|earthen

RESERVOIR 43605 Reservoir Type|disposal-tailings pond; Construction Material|unspecified

RESERVOIR 43606 Reservoir Type|disposal-unspecified; Construction Material|unspecified

RESERVOIR 43607 Reservoir Type|evaporator; Construction Material|unspecified

RESERVOIR 43608 Reservoir Type|swimming pool; Construction Material|nonearthen

RESERVOIR 43609 Reservoir Type|treatment-cooling pond; Construction Material|unspecified

RESERVOIR 43610 Reservoir Type|treatment-filtration pond; Construction Material|unspecified

RESERVOIR 43611 Reservoir Type|treatment-settling pond; Construction Material|unspecified

RESERVOIR 43612 Reservoir Type|treatment-sewage treatment pond; Construction Material|unspecified

RESERVOIR 43613 Reservoir Type|water storage; Construction Material|nonearthen; Cover Status|covered; Hydrographic Category|unspecified

RESERVOIR 43614 Reservoir Type|water storage; Construction Material|earthen; Cover status|not covered; Hydrographic Category|intermittent

RESERVOIR 43615 Reservoir Type|water storage; Construction Material|earthen; Cover status|not covered; Hydrographic Category|perennial

RESERVOIR 43616 Reservoir Type|water storage; Construction Material|nonearthen; Cover status|not covered; Hydrographic Category|unspecified

RESERVOIR 43617 Reservoir Type|water storage; Construction Material|unspecified; Cover status|unspecified; Hydrographic Category|unspecified

RESERVOIR 43618 Reservoir Type|unspecified; Construction Material|earthen

RESERVOIR 43619 Reservoir Type|unspecified; Construction Material|nonearthen

RESERVOIR 43620 Reservoir Type|unspecified; Construction Material|unspecified

ROCK 44100 feature type only: no attributes

ROCK 44101 Relationship to Surface|abovewater

ROCK 44102 Relationship to Surface|underwater

SEA/OCEAN 44500 feature type only: no attributes

SEA/OCEAN 44501 Sea/Ocean Category|reef pool

SEA/OCEAN 44502 Sea/Ocean Category|unspecified

SINK/RISE 45000 feature type only: no attributes

SNAG/STUMP 45200 feature type only: no attributes

SNAG/STUMP 45201 Snag/Stump Type|snag; Relationship to Surface|abovewater

SNAG/STUMP 45202 Snag/Stump Type|snag; Relationship to Surface|underwater

SNAG/STUMP 45203 Snag/Stump Type|stump; Relationship to Surface|abovewater

SNAG/STUMP 45204 Snag/Stump Type|stump; Relationship to Surface|underwater

SOUNDING DATUM LINE


SOUNDING DATUM LINE

50301 Positional Accuracy|approximate

SOUNDING DATUM LINE

50302 Positional Accuracy|definite

SPECIAL USE ZONE 45400 feature type only: no attributes

SPECIAL USE ZONE 45401 Special Use Zone Type|dump site; Operational Status|operational

SPECIAL USE ZONE 45402 Special Use Zone Type|dump site; Operational Status|abandoned

SPECIAL USE ZONE 45403 Special Use Zone Type|spoil area; Operational Status|operational

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Feature Type FCODE Description SPECIAL USE ZONE 45404 Special Use Zone Type|spoil area; Operational Status|abandoned

SPECIAL USE ZONE LIMIT



53301 Positional Accuracy|definite


53302 Positional Accuracy|indefinite

SPILLWAY 45500 feature type only: no attributes

SPRING/SEEP 45800 feature type only: no attributes

SPRING/SEEP 45801 Water Characteristics|unspecified

SPRING/SEEP 45802 Water Characteristics|alkaline

SPRING/SEEP 45803 Water Characteristics|hot

SPRING/SEEP 45804 Water Characteristics|sulphur

STREAM/RIVER 46000 feature type only: no attributes

STREAM/RIVER 46001 Hydrographic Category|intermittent; Positional Accuracy|definite

STREAM/RIVER 46002 Hydrographic Category|intermittent; Positional Accuracy|indefinite

STREAM/RIVER 46003 Hydrographic Category|intermittent; Positional Accuracy|not applicable

STREAM/RIVER 46004 Hydrographic Category|perennial; Positional Accuracy|definite

STREAM/RIVER 46005 Hydrographic Category|perennial; Positional Accuracy|indefinite

STREAM/RIVER 46006 Hydrographic Category|perennial; Positional Accuracy|not applicable

SUBMERGED STREAM 46100 feature type only: no attributes

SWAMP/MARSH 46600 feature type only: no attributes

TUNNEL 47800 feature type only: no attributes

UNDERPASS 48100 feature type only: no attributes

WALL 48300 feature type only: no attributes

WALL 48301 Wall Type|general case

WALL 48302 Wall Type|sea

WASH 48400 feature type only: no attributes

WATER INTAKE/OUTFLOW



48501 Water Intake/Outflow Type|intake


48502 Water Intake/Outflow Type|outflow

WATERFALL 48700 feature type only: no attributes

WELL 48800 feature type only: no attributes

WELL 48801 Product|heat

WELL 48802 Product|water; Flow Status|flowing; Water Characteristics|unspecified

WELL 48803 Product|water; Flow Status|unspecified; Water Characteristics|alkaline

WELL 48804 Product|water; Flow Status|unspecified; Water Characteristics|hot

WELL 48805 Product|water; Flow Status|unspecified; Water Characteristics|sulphur

WELL 48806 Product|water; Flow Status|unspecified; Water Characteristics|unspecified

WRECK 49200 feature type only: no attributes

WRECK 49201 Relationship to Surface|abovewater; Abovewater Portion|hull and/or superstructure

WRECK 49202 Relationship to Surface|abovewater; Abovewater Portion|mast and/or funnel

WRECK 49203 Relationship to Surface|underwater

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Appendix C. Encoding characteristics using field names Fields provide another text-based means of encoding characteristics and values. Each field encodes the value for one characteristic. The name of each field is an abbreviation of the name of the corresponding characteristic. The entries for a field are the values for the characteristic.

For example, a feature classified as "Dam/Weir" may also have the characteristic of "Construction Material" with the values of "earthen" and "nonearthen", and the characteristic of "Operational Status" with the values of "operational" and "under construction". Using fields for characteristics, the following fields and entries can be encoded:

Characteristic: COM

(Construction Material) OPS

(Operational Status)

<no entry> <no entry> earthen Operational earthen under construction

nonearthen Operational Values:

nonearthen under construction

Table 1C contains the names of fields and the corresponding characteristics.

Table 1C. Field names used for characteristics and related domains of values, listed alphabetically by field name (Note that several characteristics were not used in the initial release of the NHD. Appendix A lists feature types and characteristics that were not encoded. The "Standards for National Hydrography Dataset" (USGS, 1999) contains the names and definitions of all characteristics and values. It also lists the values that can be applied for each feature type. The document is available online through http://mapping.usgs.gov/standards/.)

Field Name Characteristic Name Domain of Values

ABW Abovewater Portion hull and/or superstructure mast and/or funnel

ANT Anchorage Type explosives isolation quarantine seaplane

CDY Canal/Ditch Type aqueduct unspecified

COM Construction Material earthen nonearthen unspecified

COS Cover Status covered not covered unspecified

FLO Flow Status flowing unspecified

GTT Gate Type

drydock floodgate lock tidegate unspecified

CGC Glaciation Category ice shelf inland ice sheet

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HZT Hazard Zone Category

crib area foul ground mine danger area piling area platform area reef area rock area shoal snag/stump area unspecified well area wreckage

HYC Hydrographic Category intermittent perennial unspecified

IMC Ice Mass Category

alpine glacier continental glacier pack ice snowfield unspecified

IAT Inundation Area Type

debris basin dewatering area duck pond general case percolation basin retarding basin

ICS Inundation Control Status controlled not controlled

OPS Operational Status abandoned operational under construction

PIT Pipeline Type

aqueduct general case penstock siphon

POA Positional Accuracy

approximate definite indefinite not applicable

POT Post Type dolphin piling

PRD Product heat water

RTS Relationship to Surface

abovewater at or near elevated underground underwater unspecified

RET Reservoir Type

aquaculture decorative pool disposal-tailings pond disposal-unspecified evaporator swimming pool treatment-cooling pond treatment-filtration pond treatment-settling pond treatment-sewage treatment pond unspecified water storage

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SOC Sea/Ocean Category reef pool unspecified

STT Snag/Stump Type snag stump

SZT Special Use Zone Type dump site spoil area

WAT Wall Type general case sea

WAC Water Characteristics

alkaline hot salt sulphur unspecified

WIT Water Intake/Outflow Type intake outflow

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Appendix D. Development of Reach Files and related concepts (Adapted from "The U.S. EPA Reach File Version 3.0 alpha release (RF-3 Alpha) technical reference" (USEPA, 1994).)

Purpose and approach In the 1970's, the U.S. Environmental Protection Agency (USEPA) began creating a series of hydrographic databases of the surface waters of the conterminous United States and Hawaii. The structure and content of these Reach File databases were created to establish hydrologic sequencing, to perform hydrologic navigation for modeling applications, and to provide unique identifiers, called reach codes, for each surface water feature.

A key characteristic of the Reach Files is the use of relations among reaches to define a connected hydrographic network. These relations provide connectivity among reaches even if their delineations do not form a topologically connected set of lines. The direction of water flow is also encoded using these relations. This connectivity enables hydrologic sequencing of reaches (identifying what is upstream and downstream of a given point in the hydrographic network), as well as navigating the network in an upstream or downstream direction.

The unique reach codes assigned to each reach have been used to geocode the locations of many water-related observations. Examples include USEPA national databases for surface waters, such as STORET Water Quality Sampling Sites, Municipal and Industrial Facility Discharges, and Drinking Water Intakes. Using the reach code, observations in these databases can be associated with their locations on surface water features.

Reach file development The Reach File was first conceived in the early 1970's with a proof-of-concept file, known as Reach File Version 1.0 Alpha (RF1A), completed in 1975. The first full implementation, referred to as “Reach File Version 1.0” (RF1), was completed in 1982. The source for RF1 was the USGS 1:250,000-scale hydrography that had been photoreduced to a scale of 1:500,000 by the National Oceanic and Atmospheric Administration. RF1 consists of approximately 68,000 reach segments comprising 650,000 miles of stream.

Although RF1 still supports broad-based national applications, the need to provide a more detailed hydrologic network motivated the development of Reach File Version 2.0 (RF2) in the late 1980's. RF2 was created by using the Feature File (now called the National Geographic Names Database) of the USGS Geographic Names Information System (GNIS) to add one new level of reach segments to RF1. RF2 contains 170,000 reach segments.

Widespread interest in providing a more comprehensive, nationally consistent hydrologic database led to the development of the Reach File Version 3-Alpha (RF3-Alpha). This database combines data from RF1 and RF2, the GNIS, and the 1988 edition of USGS 1:100,000-scale digital line graph (DLG) hydrography data. RF3-Alpha contains nearly 3,200,000 individual hydrographic features (reaches) and more than 93,000,000 coordinate points.

The NHD is the fourth in the series of continuing improvements to reach data. This database supplements attribute-based connectivity with reach delineations that provide spatial connectivity, common classification of features that underlie the reaches, and a design that encourages cooperative data maintenance and improvements among many organizations.

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Appendix E. Transport reach delineation rules and examples A transport reach represents the pathway for the movement of water through a drainage network and encodes the direction in which water flows along the reach. Transport reaches provide a basis through which locations of observations can be geocoded and linked to the drainage network. Encoding flow relations among the reaches provides the ability to navigate through a drainage network and is the basis for determining hydrologic sequencing in the network.

Lines delineate transport reaches. Linear delineations of the feature types canal/ditch, pipeline, and stream/river provide the lines for most transport reach delineations.

By themselves, however, these features may not depict all flows through a drainage network. For areal features of the type area of complex channels, estuary, ice mass, lake/pond, playa, reservoir, swamp/marsh, wash, canal/ditch, and stream/river, “artificial path" features complete the network to represent the flow of water into, through, and out of features that are delineated by using areas. "Connector" features fill gaps in the delineation of features through which water flows.

For transport reaches for which the direction of flow is known, the lines are oriented in the direction of the flow of water. Note that the direction of flow is not always known (for example, when source materials are ambiguous) or uniform (for example, in tidal areas), and so the lines are not always oriented in the direction of flow.

Three general rules determine the location of the ends of transport reaches: • the underlying feature rule, • the confluence-to-confluence rule, and • the branched path rule.

These rules govern the delineation of most transport reaches and make the delineation of most reaches a straightforward process. The rules do have exceptions, however. Unusual configurations of features require modification of the rules, as do the variable condition and ambiguities of information sources. In places where unusual configurations of feature or ambiguities in sources predominate, a larger percentage of reaches using modified rules will be encountered. This appendix describes the conditions that most often influence transport reach delineation.

Underlying feature rule The delineation of a transport reach follows those of one or more linear features. Where two or more features are followed, a transport reach follows delineations of :

• features of the same feature type (for example, exclusively stream/river) or • features of the type of canal/ditch or stream/river, in combination with features of types artificial

path and (or) connector. For an artificial path and a linear canal/ditch or stream/river to be combined to form a reach, the areal feature through which the artificial path is delineated must be of the same type as the linear feature to which the artificial path connects. For example, an artificial path through an areal canal/ditch can be combined with a linear canal/ditch to create one reach

• each feature of the type pipeline is formed into a reach. Pipelines are never combined with artificial paths to form reaches.

Figure 1E illustrates the delineation of a reach that follows features of the same type, or, in the case of the artificial path, acts as surrogates for the same type. Different reaches follow features of different types (see Figure 2E).

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Figure 1E. Transport reach delineation for features of the same type.

Figure 2E. Transport reach delineation for features of different types.

A transport reach always follows the entire delineation of the underlying feature or features; the delineation of a feature is not split among reaches. Transport reaches abut and do not overlap.

Exception: Insignificant lake/pond features (10-acre rule) An exception to the underlying feature rule is made for artificial paths that represent flow through lake/pond features of 10 acres (4.05 hectares) or less. In these cases, the lake/pond feature is ignored for

Underlying feature rule (same type, including surrogates of the same type):

REACH 1 follows STREAM/RIVER A and B, and ARTIFICIAL PATH C (which acts as a surrogate for the areal stream/river).

1A

1B

1CStream/River (intermittent)

Stream/River (perennial)

Artificial path

Stream/River

1A

2B

3C

2B

1A

2B

1A

Stream/River

Artificial path

Lake/Pond

Direction of flow

1A

2B

Canal/Ditch

Pipeline

Underlying feature rule (different types):REACH 1 follows CANAL/DITCH A REACH 2 follows PIPELINE B. Underlying feature rule (different types;

head lake/pond): REACH 1 follows ARTIFICIAL PATH A

(which acts as a surrogate for the lake/pond).

REACH 2 follows STREAM/RIVER B. Underlying feature rule (different types;

inline lake/pond): REACH 1 follows STREAM/RIVER A. REACH 2 follows ARTIFICIAL PATH B


REACH 3 follows STREAM/RIVER C. Underlying feature rule (different types;

terminal lake/pond): REACH 1 follows STREAM/RIVER A. REACH 2 follows ARTIFICIAL PATH B


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the purpose of reach delineation. The artificial path is treated as an extension of the inflowing and outflowing linear feature and is not designated as a separate transport reach (see Figure 3E).

Figure 3E. Transport reach delineation for insignificant lake/pond features.

Confluence-to-confluence rule

Like feature types (and their surrogates) In the "confluence-to-confluence" rule, a transport reach is a stretch of water between:

• confluences, • a head and a confluence, • a confluence, and a mouth or terminus, or • a head, and a mouth or terminus.

This rule applies where the features are of the same type or are of types (that is, artificial path and connector) that act as surrogates for features of the same type. These reaches must be contiguous and may not branch (see Figure 4E).

1A

1B

1CStream/River

Artificial path

Lake/Pond (10 acres or smaller)

Underlying feature rule (insignificant lake/pond):

REACH 1 follows STREAM/RIVER A, ARTIFICIAL PATH B (which acts as a surrogate for the insignificant lake/pond), and STREAM/RIVER C.

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Figure 4E. Confluence-to-confluence delineation among like feature types.

Generally, divergences and confluences are treated as "confluences" for reach delineation. Separate reaches are delineated for each channel (see Figure 5E).

Figure 5E. Confluence-to-confluence delineation for multiple channels.

When an areal feature of type stream/river splits into two channels, a decision must be made as to whether or not reach delineation (that is, the addition of artificial paths and creation reaches) should be performed in the secondary channel. The following rules (see Figure 6E) guide reach delineation for the channels:

Confluence-to-confluence rule (divergences) REACH 1 follows STREAM/RIVER A. REACH 2 follows STREAM/RIVER B. REACH 3 follows STREAM/RIVER C. REACH 4 follows STREAM/RIVER D. REACH 5 follows STREAM/RIVER E. REACH 6 follows STREAM/RIVER F.

Stream/River Confluence or divergence

1A

2B

3C

4D

5E 6F

Confluence-to-confluence rule REACH 1 follows STREAM/RIVER A. REACH 2 follows STREAM/RIVER B. REACH 3 follows STREAM/RIVER C. REACH 4 follows STREAM/RIVER D. REACH 5 follows STREAM/RIVER E. Confluence-to-confluence rule REACH 1 follows STREAM/RIVER A and

ARTIFICIAL PATH B (which acts as a surrogate for the stream/river).

REACH 2 follows ARTIFICIAL PATH C (which acts as a surrogate for the stream/river).

REACH 3 follows ARTIFICIAL PATH D (which acts as a surrogate for the stream/river).

REACH 4 follows STREAM/RIVER H and ARTIFICIAL PATH G (which acts as a surrogate for the stream/river).

REACH 5 follows STREAM/RIVER F and ARTIFICIAL PATH E (which acts as a surrogate for the stream/river).

1A

3C

2B

4D 5E

Reach

Head Terminus Confluence

Stream/River Artificial path Stream/River

Confluence

1A

3D 2C

4H 5F

4G

1B 5E

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• Select the primary channel. The channel with the most tributaries or, if tributaries are not present, the widest channel is designated as the primary channel and reaches always should be delineated on this channel.

• Decide if reaches should be delineated on the secondary channel. The size and shape of the land that separates the channels and the presence of tributaries on the secondary channel help determine if the secondary channel is significant (and should have a reach delineated). If the land separating the channels is small or if there are no tributaries, the secondary channel is minor (and need not have reaches delineated).

Figure 6E. Transport reach delineation for multiple divergent channels.

When the land separating the channels is large or long, it may be clear that the primary and the secondary channels are different stream/rivers and, therefore, both channels have reaches delineated. Regardless of the size and shape of the land, the presence of tributaries to the secondary channel weighs heavily in determining that it will have reaches delineated.

Exception: Insignificant confluences (5-mile rule) Where a simple14 tributary or divergence of less than 5 miles (approximately 8.06 kilometers) in length intersects an areal feature of type stream/river, the intersection is not considered to be a confluence for delineating the reach along the areal stream/river (see Figure 7E). This approach avoids breaking major stream/rivers into unnecessarily small reaches caused by the confluence of small tributaries and divergences.

14 A simple tributary or divergence is a reach that has no tributaries.

Stream/River

Artificial path

Stream/River

1A

2B

4D

6F 3C

7G

1A

5E

8J

2B

7H 8I

4E 4D

5G 5F

3C

Confluence-to-confluence rule (both channels significant)

REACH 1 follows ARTIFICIAL PATH A. REACH 2 follows ARTIFICIAL PATH B. REACH 3 follows ARTIFICIAL PATH C. REACH 4 follows ARTIFICIAL PATH D. REACH 5 follows ARTIFICIAL PATH E. REACH 6 follows ARTIFICIAL PATH F. REACH 7 follows STREAM/RIVER G and

ARTIFICIAL PATH H. REACH 8 follows STREAM/RIVER I and

ARTIFICIAL PATH J. Confluence-to-confluence rule (one channel

significant) REACH 1 follows ARTIFICIAL PATH A. REACH 2 follows ARTIFICIAL PATH B. REACH 3 follows ARTIFICIAL PATH C. REACH 4 follows STREAM/RIVER D and

ARTIFICIAL PATH E. REACH 5 follows STREAM/RIVER F and

ARTIFICIAL PATH G.

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Figure 17. Significant and insignificant confluences in reach delineation.

Unlike feature types: stream/river and canal/ditch The intersection of features types stream/river and canal/ditch that occurs at the same elevation (see Figure 8E) generally is treated as a confluence for the canal/ditch but not for the stream/river. An alternate approach treats the intersection as a confluence for both feature types.

Figure 8E. Confluence of stream/river and canal/ditch.

No confluence: underpasses and pipelines Because fluids carried by features do not flow together at an underpass (where features intersect at different elevations), there is no confluence, and so the confluence-to-confluence rule does not apply and the reaches are not broken at the intersection.

1A

2B

3C 4D 5E

5F

Stream/River

Canal/Ditch

3C 4D 5E

6F

Confluence-to-confluence rule (preferred approach: stream/river with canal/ditch):

REACH 3 follows CANAL/DITCH C. REACH 4 follows CANAL/DITCH D. REACH 5 follows STREAM/RIVER E and F. Other confluence-to-confluence delineations: REACH 1 follows CANAL/DITCH A. REACH 2 follows CANAL/DITCH B. Confluence-to-confluence rule (alternate

approach: stream/river with canal/ditch): REACH 3 follows CANAL/DITCH C. REACH 4 follows CANAL/DITCH D. REACH 5 follows STREAM/RIVER E. REACH 6 follows STREAM/RIVER F.

Confluence-to-confluence rule (significant and insignificant tributaries and divergences):

REACH 1 follows ARTIFICIAL PATH A. REACH 2 follows ARTIFICIAL PATH B. REACH 3 follows ARTIFICIAL PATH C.

(Reaches 1 through 3 have confluences with significant tributaries and divergences.)

REACH 4 follows ARTIFICIAL PATHS D, E, F, and G (has confluences with only insignificant tributaries and divergences).

REACH 5 follows ARTIFICIAL PATH H, STREAM/RIVER I, and ARTIFICIAL PATH J.

REACH 6 follows STREAM/RIVER K and ARTIFICIAL PATH L.

REACH 7 follows ARTIFICIAL PATH M, STREAM/RIVER N, and ARTIFICIAL PATH O.

REACH 8 follows STREAM/RIVER P and ARTIFICIAL PATH Q.

Stream/River

Artificial path

Stream/River

1A 5H 6K

7M

2B

3C

4D 4E

4F 4G

5I

5J 6L

7N

7O

8P

8Q

Shorter than 5 miles

Longer than 5 miles

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With the information available during the development of the initial release of the NHD, it was not possible to determine if fluids carried by pipelines flow together with those carried by other features (for example, stream/river and canal/ditch) with which a pipeline intersects on a map. The assumption is that the fluids do not flow together. So there is no confluence, the confluence-to-confluence rule does not apply, and the reaches are not broken at the intersection. (When it is known that a pipeline exchanges fluids with another feature of a different type, it will be treated as a confluence of unlike feature types.)

Branched path rule A branched path transport reach connects reaches that enter and exit an areal feature (see Figure 9E). The branched path transport reach avoids the need to define flow channels, confluences and divergences, and confluence-based transport reaches in areal features. It is used where information needed to delineate these items reliably is not available and in other places at the analyst’s discretion.

Reaches that follow this rule occur most often in large15 features of type lake/pond and swamp/marsh and may occur in other areal features. Artificial paths delineated within the areal feature provide the lines needed to delineate this special transport reach. The reach may branch and, infrequently, may be discontiguous (see Figure 10E).

Figure 9E. Branched path reach delineation.

In cases where channels and confluences are delineated, confluence-based reaches are delineated, and branched path transport reaches are not needed. If the flow channels delineated within a feature are incomplete, a branched path transport reach is added to provide connectivity (see Figure10E).

15 Large areal features cover 10 or more acres (approximately 4.05 or more hectares).

Branched path rule REACH 1 follows ARTIFICIAL PATHS

A, B, C, D, and E. Confluence-to-confluence rule REACH 2 follows STREAM/RIVER F. REACH 3 follows STREAM/RIVER G. REACH 4 follows STREAM/RIVER H. REACH 5 follows STREAM/RIVER I.

2F

3G 4H

1A

1D

1B

5I

1C

1E

Feature with area of 10 acres or larger.

Branched path

Other reaches

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Figure 10E. Use of branched path reaches where other features provide flow channels through areal features.

Exception: Insignificant lake/pond features (10-acre rule) An exception to the branched path rule is made for artificial paths that represent flow through lake/pond features of 10 acres (4.05 hectares) or less. In these cases, the artificial paths are treated as extensions of the inflowing and outflowing linear features, and the confluence-to-confluence delineation rule is used for transport reaches within the lake/pond (see Figure11E).

Figure 11E. Transport reach delineation in significant and insignificant lake/pond features.

Stream/River

Artificial path

Lake/Pond

1A 1B

3E

3F

2D

2C

1A 4B

4E

3F

4D

2C

Confluence-to-confluence rule (lake/pond 10 acres or smaller)

REACH 1 follows STREAM/RIVER A and ARTIFICIAL PATH B.

REACH 2 follows STREAM/RIVER C and ARTIFICIAL PATH D.

REACH 3 follows ARTIFICIAL PATH E and STREAM/RIVER F.

Branched path rule (lake/pond larger than

10 acres) REACH 1 follows STREAM/RIVER A. REACH 2 follows STREAM/RIVER C. REACH 3 follows STREAM/RIVER F. REACH 4 follows ARTIFICIAL PATHS B, D, and E.

Confluence-to-confluence rule REACH 1 follows STREAM/RIVER A. REACH 2 follows STREAM/RIVER B. REACH 3 follows STREAM/RIVER C. Branched path rule NONE. The stream/river features provide the

basis for confluence-to-confluence reaches. Confluence-to-confluence rule REACH 1 follows STREAM/RIVER A. REACH 2 follows STREAM/RIVER B. REACH 3 follows STREAM/RIVER C. Branched path rule (discontiguous reach) REACH 4 follows ARTIFICIAL PATHS D and E

(which act as a surrogate for the swamp/marsh).

1A 2B

3C

1A

1A (continued)

4E 3C

2B

4D

Swamp/Marsh

Stream/River

Artificial path (Branched path reach)

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Appendix F. Organization and examples of hydrologic units (Adapted from “Hydrologic unit maps" (Seaber, Kapinos, and Knapp, 1987)).

A hydrologic unit is a member of the hierarchical system for identifying and subdividing river-basin units of the United States. Such units are used by water resource organizations for locating, storing, retrieving, and exchanging hydrologic data; for indexing and inventorying hydrologic data in cataloging water-data acquisition activities; and in a variety of other applications.

A standard four-level hierarchy16 of hydrologic units exists for the Nation. The levels in this hierarchy, listed from largest to smallest in area, are region, subregion, accounting unit, and subbasin:

• Regions contain either the drainage area of a major river, such as the Missouri region, or the combined drainage areas of a series of rivers, such as the Texas-Gulf region, which includes a number of rivers draining into the Gulf of Mexico. There are 21 regions: 18 for the conterminous United States, and 1 each for Alaska, Hawaii, and Puerto Rico and other outlying Caribbean areas.

• The second level of classification divides the 21 regions into 222 subregions. A subregion includes the area drained by a river system, a part of a river and the tributaries to that part, one or more closed basin(s), or a group of streams forming a coastal drainage area.

• The third level subdivides many of the subregions into accounting units. These 379 units nest within, or are equivalent to, the subregions.

• The fourth level of classification is the subbasin. This hydrologic unit is a geographic area representing part or all of a surface drainage basin, a combination of drainage basins, or a distinct hydrologic feature. These units subdivide the subregions and accounting units into 2,267 areas for the Nation. They have been defined so that most subbasins are larger than 700 square miles (1,813 square kilometers) in area. In special circumstances, areas smaller than 700 square miles are identified.

All boundaries of units within the United States are hydrologic (hydrographic) in nature. At the international boundaries of the United States with Mexico and Canada, region and subregion boundaries end or coincide with the boundary. The boundaries of accounting units and subbasins are hydrologic, and can be extended into Mexico and Canada. With the exception of areas near the international boundary, each unit nests within, or coincides with, its next largest unit. Seaber, Kapinos, and Knapp (1987) provide additional technical criteria for delineating hydrologic units.

Each hydrologic unit has been assigned a name corresponding to the principal hydrologic feature(s) within the unit. In the absence of such features, the assigned name may reflect a cultural or political feature within the unit. All regions and subregions are uniquely named; however, the accounting units are named uniquely only within each region, and the subbasins are named uniquely only within each accounting unit. Duplication of some names at the subbasin level is unavoidable because a large number of streams share the same names.

Shown below is an example of the units in the hierarchy (see Figure 1F):

region — New England region: the drainage within the United States that ultimately discharges into: (a) the Bay of Fundy, (b) the Atlantic Ocean within and between the States of Maine and Connecticut, (c) Long Island Sound north of the New York-Connecticut state line, and (d) the Riviere St. Francois, a tributary of the St. Lawrence River.

subregion — Connecticut: the Connecticut River basin. accounting unit — Lower Connecticut: the Connecticut River basin below Vernon Dam. subbasin — Chicopee.

16 Efforts are under way to add levels of subdivision, especially fifth- and sixth-level units.

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Figure 1F. Hydrologic units in the New England region. (Data from Allord, 1992).

New England region

Connecticut subregion

Lower Connecticut accounting unit Chicopee

cataloging unit

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An 8-digit hydrologic unit code uniquely identifies each of the four levels of classification within four 2-digit fields. The first 2 digits identify regions, the first 4 digits identify subregions, the first 6 digits identify accounting units, and the full 8 digits identify subbasins. For example, the hydrologic unit codes for the example provided above are as follows:

01 — the region (New England) 0108 — the subregion (Connecticut) 010802 — the accounting unit (Lower Connecticut) 01080204 — the subbasin (Chicopee)

Zeroes in the two-digit accounting unit field indicate that the accounting unit and the subregion are the same. Zeroes in the two-digit subbasin field indicate that the subbasin and the accounting unit are the same.

Additional information is available in the online document "Hydrologic unit maps" (USGS, 1998) at http://water.usgs.gov/lookup/get?huc.

Hydrologic unit boundaries have also been incorporated as a map layer in the National Atlas of the United States and are available online at http://www.nationalatlas.gov/hucsm.htm.

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Appendix G. Peculiarities

The NHD contains certain peculiarities, which generally reflect inconsistencies and deficiencies in the source data used to create the NHD. The items in this category tend to be irritating, but they do not undermine the usefulness of the NHD and can be resolved on a case by case basis. In some cases, such as quad edge effects, mismatches are likely to be encountered frequently. However, developers of the NHD decided it would be more valuable to make the NHD available as soon as possible rather than investing the time and money that would have been required to identify and correct every quad edge mismatch in the raw data. The purpose of this list is to alert users to the existence of these peculiarities so that they can be anticipated and dealt with as they occur. The following summary describes the most significant of these and any steps, already taken or planned, to resolve them.

Quad Edge Effects Features in the NHD still stop and start at USGS 1:100,000-scale quadrangle edges. Reaches, however, are continuous across quad edges. By delineating features based on the 1:100,000-scale quadrangle, the NHD maintains the lineage of each feature. By making the reaches continuous across quad edges, the NHD maintains the ability to navigate through the hydrologic network and the ability to create routes and event data without artificial breaks at the quad edges. Because the NHD is designed to manage the data hydrologically, discrepancies at quad edges become more obvious. Some features stop abruptly, and some features cross quad edges but are classified differently on both sides. This reflects the different sources and dates used to make the 1:100,000-scale maps and associated DLGs from which the NHD was created.

DLG-3 coding inconsistencies The feature types and characteristics in the NHD reflect the coding of the original DLGs. If a feature was miscoded in the DLG, it likely will be misclassified in the NHD. In some cases, the DLG did not provide enough information to create a correct NHD feature. The following are a few “systematic” coding problems we would like to point out.

Inland oceans The DLG code 050 0116 is defined as “bay, estuary, gulf, ocean, or sea,”. This code was intended to describe saltwater areas, but in some cases it was used for “named” bays within freshwater areas. The DLG code 050 0116 was always converted to the NHD feature sea/ocean even when applied to an inland “bay”. Therefore, some landlocked (i.e.inland) sea/ocean features were created.

Disguised aqueducts Conduits like penstocks, aqueducts, tunnels, and pipelines were coded in various ways in the DLGs. In the NHD, the intent is to classify these features by their form. Due to coding inconsistencies or inadequate information, the DLG did not always provide enough information to carry out the NHD intent. For example, in the NHD an open artificial channel would be classified as a canal/ditch, but a closed conduit would be classified as a pipeline. If the feature is “underground,” it would have a “relationship to surface” of underground, but if it is inside a tunnel, then both canal/ditch and tunnel would have been created in the NHD. The same real-world feature also could be classified differently if it appeared on different 1:100,000-scale quads (see previous quad-edge effects discussion)

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Coastal features - foreshore versus sand Several symbols were used on the topographic and topographic/bathymetric maps to identify sand, sand dunes, tidal flats, and foreshore areas. Often these symbols were confused when applying DLG codes. As a result, foreshore features sometimes stop abruptly (see previous quad-edge effects discussion).

Stream/rivers controlled by dams that become a series of slackwater pools When a stream/river is impounded or has the flow controlled for navigation, it often is difficult to classify the feature based on the information shown on the topographic map. Does it behave like a stream/river or more like a lake/pond? Most often, if a feature was named on the map as a ‘lake’, it was coded in DLG as a lake. But this was done inconsistently. It becomes more obvious in the NHD because reaches are delineated differently if the feature is a stream/river or a lake/pond.

Reservoir versus lake/pond In the source DLG data, the decision to classify a waterbody as a lake/pond or a reservoir was based on the name of the feature. If the name contained the generic term “reservoir”, it was coded 050 0101 (reservoir). In the NHD, an attempt was made to clarify this distinction based on the form of the feature. However, in some cases there was not enough information in the DLG to make the correct classification. A lake/pond is a standing body of water with a predominately natural shoreline (it may be impounded). A reservoir is a constructed basin. In building the NHD, if the feature was coded as 050 0101(reservoir) or 050 0421(lake/pond) it was classified as a lake/pond unless the geometry of the feature indicated that it was a constructed basin (it was square). Other DLG codes for sewage disposal pond, filtration pond, and aquaculture ponds were converted to the NHD feature reservoir.

NHD linework doesn’t match the published USGS 1:100,000-scale map Some of the 1:100,000-scale DLGs were created from BLM editions or from earlier planimetric editions of the1:100,000-scale maps. These maps were replaced with USGS topographic editions, but usually, the DLG was not re-digitized. The date of the source DLG indicates which edition of the map was used for digitizing. If the date of the source DLG is older than the published 1:100,000-scale topographic map, it is likely that the DLG was digitized from a planimetric or BLM edition.

Names Names for transport reaches came from RF3. Names from RF3 were validated against GNIS. There are reaches that have names in GNIS that do not have names in the NHD and some names in NHD are applied to the wrong path when compared to GNIS. Users are encouraged to supply updates to names to the USGS. GNIS is continually updated (including Board of Geographic Names (BGN) decisions and new names), therefore, names in the NHD are likely to be more current than those on published 1:100,000-scale maps.

CU boundaries and the features that touch and cross them The NHD is a seamless dataset. However, initial distribution of the NHD data is by subbasins. It is not possible to “clip” a subbasin from the seamless data using existing subbasin boundaries, because the resolution of the CU boundaries does not match the resolution of the reach linework. Generally the CU boundaries are based on 1:250,000-scale linework and the reaches are based on 1:100,000-scale linework. Data are pulled from the seamless NHD database and associated with the appropriate CU using a number of rules.

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1. All reaches that have a reach code that includes the CU number 2. All features that are part of the reaches from step 1 (artificial path, 1-D canal/ditch, connector, 1-D stream/river, and pipeline) 3. All non-reached features (including landmarks and 2-D stream/rivers and 2-D canal/ditches) that are within or “touch” the CU boundary.

As a result of these rules, features can extend outside the CU boundaries. The most obvious examples are 2-D stream/rivers. They are included in a CU if they touch the CU boundary. This means they will appear in multiple adjacent subbasins. These duplicated features should not pose a problem unless adjacent subbasins are to be appended. A tool to append subbasins is being developed that will properly delete one of the instances. Some reaches may appear to cross CU boundaries. The flow direction relationships are used to determine which CU a reach is associated with, not just the CU boundary. In building the NHD, problems with reaches being clipped in RF3 were fixed whenever possible, but a few cases where reaches are associated with the wrong CU still exist.

Squared-off coastal CU boundaries Along the coast, the NHD used CU boundaries that were extended as straight lines into the ocean to the edge of the 1:100,000-scale quad to ensure that all coastal islands in the DLG were included in the NHD.

Empty Subbasins There are a number of subbasins that represent a single waterbody with the edge of the water body coinciding with the edge of the subbasin. These subbasins are: 01100007 – Long Island Sound 02040204 – Delaware Bay 02060001 – Upper Chesapeake 02080101 – Lower Chesapeake 04020300 – Lake Superior 04060200 – Lake Michigan 04080300 – Lake Huron 04120200 – Lake Erie 04150200 – Lake Ontario In addition, the NHD does not have data for: 20090000 - Northwestern Hawaiian Islands 21030001 - Canal Zone. Panama Canal Zone. 21030002 - Navassa. Navassa Island. 21030003 - Roncador-Serrana. Roncador and Serrana Banks. The waterbodies represented by these subbasins are not represented as waterbody reaches in the NHD. The boundaries of the subbasins that border these ‘waterbody subbasins’ were extended to encompass the area enclosed by the ‘waterbody subbasins’. Any hydrographic features such as islands, including streams and lakes on the islands, that are inside these waterbody subbasins are included in the bordering subbasins.

Coastline reaches that bound stream/rivers RF3 and DLG sometimes used different sources and rules in tidal areas to determine where a stream ends and the ocean begins. In the NHD, the feature classification comes form the DLG, but the information used to create coastline reaches comes from RF3. As a result, coastline reaches sometimes bound stream/river features.

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Waterbody reaches Waterbody reaches were created automatically by associating a lake/pond (and only lake/ponds) in the NHD with a ‘lake’ identified in RF3. Because of discrepancies in coding and coordinates among the various sources, not all lake/ponds in the NHD have been identified as waterbody reaches. Users needing to link information to a lake that does not have an associated reach with an assigned reach code, may create a reach for the lake/pond and provide the updates to the NHD. In some cases, lake/ponds near the CU boundary may appear not to be associated with a waterbody reach. Because of the rules needed to create subbasins from the seamless NHD database, waterbodies along a CU boundary may appear in both subbasins. The waterbody reaches however, will only appear in the CU identified by the waterbody reach code.

Flow/coordinate direction/measure direction In the NHD, flow direction is known and recorded for some, but not all, transport reaches. Flow direction is recorded in flow relationships that describe which transport reaches exchange flow. Recording the flow direction for a transport reach also affects the order of the reach’s coordinates and the direction of the measures along the reach’s route. When flow direction is recorded for a transport reach, the coordinates of the reach are ordered from upstream to downstream and the measures along the routes for the reach run from downstream to upstream. When flow direction is either unknown or not recorded for a transport reach, coordinate order and measure direction are indeterminate. The coordinates of coastal reaches are ordered so that the water is to the right and the land is to the left and the measures along the routes run the opposite direction. For more information on this subject, see the Introducing the NHDinARC technical reference.

Artificial Paths that fall outside of the 2-D features they represent Artificial paths were created in 2-D stream/rivers, 2-D canal/ditches, and lake/ponds using ARC/INFO grid with a tolerance of approximately 17 meters. Artificial paths may fall outside of the feature they represent, they may fall on top of an edge of the feature they represent, or they may fall extremely close to the edge of a feature they represent. Artificial paths were only created in headwater lake/ponds when the direction of flow was available from RF3. Artificial paths do not follow submerged streams, which represent the old stream channel through an impounded lake/pond.

Documents

The National Hydrography Dataset National Hydrography Dataset ... Reach code ... Encoding flow relations among transport and coastline reaches